From c489c440258c0e764d9a9424d724dd6306c8bd59 Mon Sep 17 00:00:00 2001
From: Karl Berry <karl@freefriends.org>
Date: Thu, 5 Apr 2018 21:11:06 +0000
Subject: unravel (5apr18)

git-svn-id: svn://tug.org/texlive/trunk@47308 c570f23f-e606-0410-a88d-b1316a301751
---
 Master/texmf-dist/doc/latex/unravel/unravel.pdf    |  Bin 709793 -> 688977 bytes
 Master/texmf-dist/source/latex/unravel/unravel.dtx | 1147 ++++++++++++--------
 Master/texmf-dist/tex/latex/unravel/unravel.sty    |  743 ++++++++-----
 3 files changed, 1141 insertions(+), 749 deletions(-)

diff --git a/Master/texmf-dist/doc/latex/unravel/unravel.pdf b/Master/texmf-dist/doc/latex/unravel/unravel.pdf
index 3dda4e56617..2fb03b08529 100644
Binary files a/Master/texmf-dist/doc/latex/unravel/unravel.pdf and b/Master/texmf-dist/doc/latex/unravel/unravel.pdf differ
diff --git a/Master/texmf-dist/source/latex/unravel/unravel.dtx b/Master/texmf-dist/source/latex/unravel/unravel.dtx
index 328b20512ed..6572f1ed1ea 100644
--- a/Master/texmf-dist/source/latex/unravel/unravel.dtx
+++ b/Master/texmf-dist/source/latex/unravel/unravel.dtx
@@ -1,5 +1,5 @@
 % \iffalse
-%% File: unravel.dtx Copyright (C) 2013, 2015 Bruno Le Floch
+%% File: unravel.dtx Copyright (C) 2013, 2015, 2018 Bruno Le Floch
 %%
 %% It may be distributed and/or modified under the conditions of the
 %% LaTeX Project Public License (LPPL), either version 1.3c of this
@@ -23,9 +23,9 @@
 %
 % \title{The \textsf{unravel} package: \\
 %   watching TeX digest tokens\thanks{This
-%     file has version number 0.2, last revised 2015/09/30.}}
+%     file has version number 0.2e, last revised 2018/04/04.}}
 % \author{Bruno Le Floch}
-% \date{2015/09/30}
+% \date{2018/04/04}
 %
 % \maketitle
 % \tableofcontents
@@ -132,8 +132,8 @@
 %
 % Given all the work that \pkg{unravel} has to do to emulate \TeX{}, it
 % is not fast on very large pieces of code.  For instance, running it on
-% |\documentclass{article}| takes about ten minutes on my machine, and
-% finishes after slightly more than $20000$ steps.
+% |\documentclass{article}| takes about one minute on my machine, and
+% finishes after somewhat less than $21000$ steps.
 % \begin{verbatim}
 % \RequirePackage{unravel}
 % \unravel{\documentclass{article}\relax}
@@ -148,12 +148,11 @@
 % would if your file ended just after |\documentclass{article}|.  After
 % running the above through \pdfTeX{}, one can check that the result is
 % identical to that without \pkg{unravel}.  Note that
-% \cs{unravel}|\usepackage{lipsum}\relax|, despite taking as many steps
-% to complete, is four times slower, because \tn{newcommand} uses
+% \cs{unravel}|{\usepackage{lipsum}\relax}|, despite taking as many steps
+% to complete, is much slower, because \tn{newcommand} uses
 % delimited arguments, which prevent some optimizations that
 % \pkg{unravel} can otherwise obtain.  For comparison,
-% |\unravel{\lipsum[1-30]}|, which also takes $20000$ step, takes
-% $8$~minutes to complete.
+% |\unravel{\lipsum[1-30]}| also takes $20000$ step.
 %
 % \subsection{Options}
 % \label{sec:options}
@@ -344,7 +343,7 @@
 % \end{macro}
 %
 % \begin{macro}[int]{\@@_setup_save:}
-% \begin{macro}[aux, EXP]{\@@_setup_save_aux:n}
+% \begin{macro}[EXP]{\@@_setup_save_aux:n}
 %   This saves into \cs{l_@@_setup_restore_tl} the current catcodes
 %   (from $0$ to $255$ only), \tn{endlinechar}, \tn{escapechar},
 %   \tn{newlinechar}.
@@ -488,8 +487,8 @@
 % \cs{str_range:nnn}.  Otherwise loading \pkg{l3str} would give
 % an error.
 %    \begin{macrocode}
-\RequirePackage{expl3,xparse}[2015/09/11]
-\RequirePackage{gtl}[2015/09/21]
+\RequirePackage{expl3,xparse}[2018/02/21]
+\RequirePackage{gtl}[2018/04/04]
 \csname cs_if_exist:cF\endcsname{str_range:nnn}{\RequirePackage{l3str}}
 %    \end{macrocode}
 %
@@ -507,7 +506,7 @@
   {%
     \csname @@_setup_restore:\endcsname
     \ProvidesExplPackage
-      {unravel} {2015/09/30} {0.2} {Watching TeX digest tokens}%
+      {unravel} {2018/04/04} {0.2e} {Watching TeX digest tokens}%
     \csname @@_setup_unravel:\endcsname
   }%
 %    \end{macrocode}
@@ -536,10 +535,11 @@
 \cs_new_eq:NN \@@_mag:                   \tex_mag:D
 \cs_new_eq:NN \@@_nullfont:              \tex_nullfont:D
 \cs_new_eq:NN \@@_the:w                  \tex_the:D
+\cs_new_eq:NN \@@_number:w               \tex_number:D
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\c_@@_prompt_ior, \c_@@_noprompt_ior}
+% \begin{macro}{\c_@@_prompt_ior, \c_@@_noprompt_ior}
 %   These are not quite primitives, but are very low-level
 %   |ior| streams to prompt the user explicitly or not.
 %    \begin{macrocode}
@@ -552,6 +552,7 @@
 %
 % Variants that we need.
 %    \begin{macrocode}
+\cs_generate_variant:Nn \seq_push:Nn { Nf }
 \cs_generate_variant:Nn \str_head:n { f }
 \cs_generate_variant:Nn \tl_to_str:n { o }
 \cs_generate_variant:Nn \tl_if_head_eq_meaning:nNT { V }
@@ -559,9 +560,8 @@
 \cs_generate_variant:Nn \tl_if_in:nnTF { nV }
 \cs_generate_variant:Nn \tl_if_in:NnTF { No , NV }
 \cs_generate_variant:Nn \tl_if_single_token:nT { V }
-\cs_generate_variant:Nn \tl_gset_rescan:Nnn { Nnx }
 \cs_generate_variant:Nn \gtl_gput_right:Nn { NV }
-\cs_generate_variant:Nn \ior_get_str:NN { Nc }
+\cs_generate_variant:Nn \ior_str_get:NN { Nc }
 \cs_generate_variant:Nn \gtl_if_empty:NTF { c }
 \cs_generate_variant:Nn \gtl_to_str:N { c }
 \cs_generate_variant:Nn \gtl_gpop_left:NN { c }
@@ -572,17 +572,37 @@
 \cs_generate_variant:Nn \gtl_gclear_new:N { c }
 %    \end{macrocode}
 %
+% \begin{variable}{\l_@@_exp_tl}
+% \begin{macro}{\@@_exp_args:Nx, \@@_exp_args:NNx}
+%   Low-level because \cs{exp_args:Nx} redefines an internal
+%   \pkg{l3expan} variable which may be appearing in code that we debug.
+%    \begin{macrocode}
+\tl_new:N \l_@@_exp_tl
+\cs_new_protected:Npn \@@_exp_args:Nx #1#2
+  {
+    \cs_set_nopar:Npx \l_@@_exp_tl { \exp_not:N #1 {#2} }
+    \l_@@_exp_tl
+  }
+\cs_new_protected:Npn \@@_exp_args:NNx #1#2#3
+  {
+    \cs_set_nopar:Npx \l_@@_exp_tl { \exp_not:N #1 \exp_not:N #2 {#3} }
+    \l_@@_exp_tl
+  }
+%    \end{macrocode}
+% \end{macro}
+% \end{variable}
+%
 % \subsubsection{Miscellanous helpers}
 %
-% \begin{macro}[aux]{\@@_tmp:w}
+% \begin{macro}{\@@_tmp:w}
 %   Temporary function used to define other functions.
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_tmp:w { }
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_file_get:nN}
-% \begin{macro}[aux]{\@@_file_get_aux:wN}
+% \begin{macro}{\@@_file_get:nN}
+% \begin{macro}{\@@_file_get_aux:wN}
 %    \begin{macrocode}
 \cs_set_protected:Npn \@@_tmp:w #1
   {
@@ -607,7 +627,7 @@
 % \end{macro}
 %
 % \begin{macro}[int, EXP]{\@@_tl_first_int:N}
-% \begin{macro}[aux, EXP]{\@@_tl_first_int_aux:Nn}
+% \begin{macro}[EXP]{\@@_tl_first_int_aux:Nn}
 %   Function that finds an explicit number in a token list.  This is
 %   used for instance when implementing \tn{read}, to find the stream
 %   \meta{number} within the whole \tn{read} \meta{number} |to|
@@ -675,7 +695,7 @@
 % \subsubsection{String helpers}
 %
 % \begin{macro}[int]{\@@_strip_escape:w}
-% \begin{macro}[aux]{\@@_strip_escape_aux:N, \@@_strip_escape_aux:w}
+% \begin{macro}{\@@_strip_escape_aux:N, \@@_strip_escape_aux:w}
 %   This is based on the 2013-07-19 (and earlier) version of
 %   \cs{cs_to_str:N}.  There are three cases.  If the escape character
 %   is printable, the charcode test is false, and
@@ -683,8 +703,8 @@
 %   character is a space, the charcode test is true, and if there is no
 %   escape charcter, the test is unfinished after |\token_to_str:N \ |.
 %   In both of those cases, \cs{@@_strip_escape_aux:w} inserts
-%   |-\__int_value:w \fi: \c_zero|.  If the escape character was a
-%   space, the test was true, and \cs{__int_value:w} converts
+%   |-\@@_number:w \fi: \c_zero|.  If the escape character was a
+%   space, the test was true, and \cs{int_value:w} converts
 %   \cs{c_zero} to~|0|, hence the leading roman numeral expansion
 %   removes a space from what follows (it is important that what follows
 %   cannot start with a digit).  Otherwise, the test takes~|-| as its
@@ -699,17 +719,16 @@
   }
 \cs_new:Npn \@@_strip_escape_aux:N #1 { \c_zero }
 \cs_new:Npn \@@_strip_escape_aux:w #1#2
-  { - \__int_value:w #1 \c_zero }
+  { - \@@_number:w #1 \c_zero }
 %    \end{macrocode}
 % \end{macro}
 % \end{macro}
 %
 % ^^A todo: change approach: this abuses future l3obj.
 % \begin{macro}[int,EXP]{\@@_to_str:n}
-% \begin{macro}[aux,EXP]
+% \begin{macro}[EXP]
 %   {\@@_to_str_auxi:w, \@@_to_str_auxii:w, \@@_gtl_to_str:n}
 %   Use the type-appropriate conversion to string.
-%   This unavoidably uses an internal function of \pkg{gtl}.
 %    \begin{macrocode}
 \cs_new:Npn \@@_to_str:n #1
   {
@@ -728,14 +747,14 @@
     \cs_new:Npn \@@_to_str_auxii:w ##1 #1 ##2 \q_mark ##3 \q_stop
       { \cs_if_exist_use:cF { @@_ ##2 _to_str:n } { \tl_to_str:n } }
   }
-\exp_args:No \@@_tmp:w { \tl_to_str:n { s__ } }
-\cs_new:Npn \@@_gtl_to_str:n #1 { \__gtl_to_str:w #1 }
+\exp_args:No \@@_tmp:w { \tl_to_str:n { s _ _ } }
+\cs_new:Npn \@@_gtl_to_str:n { \gtl_to_str:n }
 %    \end{macrocode}
 % \end{macro}
 % \end{macro}
 %
 % \begin{macro}[int]{\@@_str_truncate_left:nn}
-% \begin{macro}[aux]{\@@_str_truncate_left_aux:nnn}
+% \begin{macro}{\@@_str_truncate_left_aux:nnn}
 %   Truncate the string |#1| to a maximum of |#2| characters.  If it is
 %   longer, replace some characters on the left of the string by
 %   |(123~more~chars)~| with the appropriate number instead of~|123|.
@@ -760,7 +779,7 @@
 % \end{macro}
 %
 % \begin{macro}[int]{\@@_str_truncate_right:nn}
-% \begin{macro}[aux]{\@@_str_truncate_right_aux:nnn}
+% \begin{macro}{\@@_str_truncate_right_aux:nnn}
 %   Truncate the string |#1| to a maximum of |#2| characters.  If it is
 %   longer, replace some characters on the right of the string by
 %   |~(123~more~chars)| with the appropriate number instead of~|123|.
@@ -827,7 +846,7 @@
 %
 % \begin{macro}[int]{\@@_token_to_char:N}
 % \begin{macro}[int]{\@@_meaning_to_char:n, \@@_meaning_to_char:o}
-% \begin{macro}[aux]
+% \begin{macro}
 %   {\@@_meaning_to_char_auxi:w, \@@_meaning_to_char_auxii:w}
 %   From the meaning of a character token (with arbitrary character
 %   code, except active), extract the character itself (with string
@@ -943,8 +962,7 @@
       {
         \gtl_if_head_is_N_type:NTF #1
           {
-            \exp_last_unbraced:Nx \@@_token_if_definable:NTF
-              { \gtl_head:N #1 }
+            \gtl_head_do:NN #1 \@@_token_if_definable:NTF
               { \prg_return_true: }
               { \prg_return_false: }
           }
@@ -957,6 +975,51 @@
 %
 % \subsubsection{Helpers for previous input}
 %
+% \begin{macro}[EXP]{\@@_prev_input_count:, \@@_prev_input_count_aux:n}
+%    \begin{macrocode}
+\cs_new_nopar:Npn \@@_prev_input_count:
+  {
+    \int_eval:n
+      {
+        0
+        \seq_map_function:NN \g_@@_prev_input_seq
+          \@@_prev_input_count_aux:n
+      }
+  }
+\cs_new:Npn \@@_prev_input_count_aux:n #1
+  { \tl_if_empty:nF {#1} { + 1 } }
+%    \end{macrocode}
+% \end{macro}
+%
+% \begin{macro}
+%   {
+%     \@@_prev_input_get:N,
+%     \@@_prev_input_gpush:,
+%     \@@_prev_input_gpush:N,
+%     \@@_prev_input_gpop:N,
+%     \@@_prev_input_gpush_gtl:,
+%     \@@_prev_input_gpush_gtl:N,
+%     \@@_prev_input_gpop_gtl:N,
+%   }
+% ^^A todo: doc and add optional sanity checking
+%    \begin{macrocode}
+\cs_new_protected_nopar:Npn \@@_prev_input_get:N
+  { \seq_get_right:NN \g_@@_prev_input_seq }
+\cs_new_protected_nopar:Npn \@@_prev_input_gpush:
+  { \seq_gput_right:Nn \g_@@_prev_input_seq { } }
+\cs_new_protected_nopar:Npn \@@_prev_input_gpush:N
+  { \seq_gput_right:NV \g_@@_prev_input_seq }
+\cs_new_protected_nopar:Npn \@@_prev_input_gpop:N
+  { \seq_gpop_right:NN \g_@@_prev_input_seq }
+\cs_new_protected_nopar:Npn \@@_prev_input_gpush_gtl:
+  { \seq_gput_right:NV \g_@@_prev_input_seq \c_empty_gtl }
+\cs_new_protected_nopar:Npn \@@_prev_input_gpush_gtl:N
+  { \seq_gput_right:NV \g_@@_prev_input_seq }
+\cs_new_protected_nopar:Npn \@@_prev_input_gpop_gtl:N
+  { \seq_gpop_right:NN \g_@@_prev_input_seq }
+%    \end{macrocode}
+% \end{macro}
+%
 % \begin{macro}[int]
 %   {
 %     \@@_prev_input_silent:n,
@@ -972,17 +1035,21 @@
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_prev_input_silent:n #1
   {
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_prev_input_tl
+    \@@_prev_input_gpop:N \l_@@_prev_input_tl
     \tl_put_right:Nn \l_@@_prev_input_tl {#1}
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_prev_input_tl
+    \@@_prev_input_gpush:N \l_@@_prev_input_tl
   }
-\cs_generate_variant:Nn \@@_prev_input_silent:n { V , x }
+\cs_generate_variant:Nn \@@_prev_input_silent:n { V }
+\cs_new_protected:Npn \@@_prev_input_silent:x
+  { \@@_exp_args:Nx \@@_prev_input_silent:n }
 \cs_new_protected:Npn \@@_prev_input:n #1
   {
     \@@_prev_input_silent:n {#1}
     \@@_print_action:x { \tl_to_str:n {#1} }
   }
-\cs_generate_variant:Nn \@@_prev_input:n { V , x }
+\cs_generate_variant:Nn \@@_prev_input:n { V }
+\cs_new_protected:Npn \@@_prev_input:x
+  { \@@_exp_args:Nx \@@_prev_input:n }
 %    \end{macrocode}
 % \end{macro}
 % \end{macro}
@@ -991,16 +1058,16 @@
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_prev_input_gtl:N #1
   {
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_prev_input_gtl
+    \@@_prev_input_gpop_gtl:N \l_@@_prev_input_gtl
     \gtl_concat:NNN \l_@@_prev_input_gtl \l_@@_prev_input_gtl #1
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_prev_input_gtl
+    \@@_prev_input_gpush_gtl:N \l_@@_prev_input_gtl
   }
 %    \end{macrocode}
 % \end{macro}
 %
 % \begin{macro}[int]{\@@_prev_input_join_get:nN}
-% \begin{macro}[aux]{\@@_join_get_aux:NNN}
-%   Pops \cs{g_@@_prev_input_seq} twice to get some value in
+% \begin{macro}{\@@_join_get_aux:NNN}
+%   Pops the previous-input sequence twice to get some value in
 %   \cs{l_@@_head_tl} and some sign or decimal number in
 %   \cs{l_@@_tmpa_tl}.  Combines them into a value, using
 %   the appropriate evaluation function, determined based
@@ -1014,14 +1081,14 @@
         { 3 } { \@@_join_get_aux:NNN \muskip_eval:n \etex_muexpr:D }
       }
       {
-        \msg_error:nnn { unravel } { internal } { join-factor }
+        \@@_error:nnnnn { internal } { join-factor } { } { } { }
         \@@_join_get_aux:NNN \use:n \prg_do_nothing:
       }
   }
 \cs_new_protected:Npn \@@_join_get_aux:NNN #1#2#3
   {
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_tmpa_tl
+    \@@_prev_input_gpop:N \l_@@_head_tl
+    \@@_prev_input_gpop:N \l_@@_tmpa_tl
     \tl_set:Nx #3 { #1 { \l_@@_tmpa_tl #2 \l_@@_head_tl } }
   }
 %    \end{macrocode}
@@ -1199,7 +1266,7 @@
 %    \begin{macrocode}
 \gtl_new:N \l_@@_head_gtl
 \tl_new:N  \l_@@_head_tl
-\token_new:Nn \l_@@_head_token { ? }
+\cs_new_eq:NN \l_@@_head_token ?
 \int_new:N \l_@@_head_cmd_int
 \int_new:N \l_@@_head_char_int
 %    \end{macrocode}
@@ -1227,7 +1294,7 @@
 % \begin{variable}{\l_@@_defined_tl, \l_@@_defining_tl}
 %   The token that is defined by the prefixed command (such as
 %   \tn{chardef} or \tn{futurelet}), and the code to define it.  We do
-%   not use the \cs{g_@@_prev_input_seq} to store that code: rather,
+%   not use the the previous-input sequence to store that code: rather,
 %   this sequence contains a string representation of the code, which is
 %   not suitable for the definition.  This is safe, as definitions
 %   cannot be nested.  This is needed for expanding assignments, as
@@ -2132,7 +2199,7 @@
 % \cs{l_@@_head_token}.
 %
 % \begin{macro}[int]{\@@_get_next:}
-% \begin{macro}[aux]{\@@_get_next_aux:w}
+% \begin{macro}{\@@_get_next_aux:w}
 %   If the input is empty, forcefully exit.  Otherwise,
 %   remove the first token in the input, and store it in
 %   \cs{l_@@_head_gtl}.  Set \cs{l_@@_head_token} equal in meaning to
@@ -2207,8 +2274,8 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_set_cmd_aux_meaning:}
-% \begin{macro}[aux,EXP]{\@@_set_cmd_aux_meaning:w}
+% \begin{macro}{\@@_set_cmd_aux_meaning:}
+% \begin{macro}[EXP]{\@@_set_cmd_aux_meaning:w}
 %   Remove the leading escape character (\cs{@@_strip_escape:w} takes
 %   care of special cases there) from the \tn{meaning} of the first
 %   token, then remove anything after the first~|:|, which is present
@@ -2238,9 +2305,9 @@
 % \end{macro}
 % \end{macro}
 %
-% \begin{macro}[aux]
+% \begin{macro}
 %   {\@@_set_cmd_aux_primitive:nTF, \@@_set_cmd_aux_primitive:oTF}
-% \begin{macro}[aux]{\@@_set_cmd_aux_primitive:nn}
+% \begin{macro}{\@@_set_cmd_aux_primitive:nn}
 %   Test if there is any information about the given (cleaned-up)
 %   \tn{meaning}.  If there is, use that as the command and character
 %   integers.
@@ -2264,7 +2331,7 @@
 % \end{macro}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_set_cmd_aux_macro:}
+% \begin{macro}{\@@_set_cmd_aux_macro:}
 %   The token is a macro.  There is no need to determine whether the
 %   macro is long/outer.
 %    \begin{macrocode}
@@ -2276,7 +2343,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_set_cmd_aux_unknown:}
+% \begin{macro}{\@@_set_cmd_aux_unknown:}
 %   Complain about an unknown primitive, and consider it as if it were
 %   \tn{relax}.
 %    \begin{macrocode}
@@ -2284,13 +2351,13 @@
   {
     \exp_last_unbraced:NV \@@_set_cmd_aux_primitive:nn
       \c_@@_tex_relax_tl
-    \msg_error:nnx { unravel } { unknown-primitive }
-      { \l_@@_head_meaning_tl }
+    \@@_error:nxxxx { unknown-primitive }
+      { \l_@@_head_meaning_tl } { } { } { }
   }
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_set_cmd_aux_cs:}
+% \begin{macro}{\@@_set_cmd_aux_cs:}
 %   If the \tn{meaning} contains \verb*|elect font|, the control
 %   sequence is \tn{nullfont} or similar (note that we do not search for
 %   \verb*|select font|, as the code to trim the escape character from
@@ -2313,13 +2380,13 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]
+% \begin{macro}
 %   {
 %     \@@_set_cmd_aux_numeric:,
 %     \@@_set_cmd_aux_numeric:w,
 %     \@@_set_cmd_aux_given:n
 %   }
-% \begin{macro}[aux,rEXP]{\@@_set_cmd_aux_numeric:N}
+% \begin{macro}[rEXP]{\@@_set_cmd_aux_numeric:N}
 %   Insert \cs{q_mark} before the first non-letter (in fact, anything
 %   less than~|A|) in the \tn{meaning} by looping one character at a
 %   time (skipping spaces, but there should be none).  We expect the
@@ -2375,8 +2442,8 @@
 % \end{macro}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_set_cmd_aux_char:}
-% \begin{macro}[aux]{\@@_set_cmd_aux_char:w}
+% \begin{macro}{\@@_set_cmd_aux_char:}
+% \begin{macro}{\@@_set_cmd_aux_char:w}
 %   At this point, the \tn{meaning} token list has been shortened by the
 %   code meant to remove the escape character.  We thus set it again to
 %   the \tn{meaning} of the leading token.  The command is then the
@@ -2393,7 +2460,7 @@
       { \tl_set:Nn \l_@@_head_meaning_tl { other~ } }
     \exp_after:wN \@@_set_cmd_aux_char:w
       \l_@@_head_meaning_tl \q_stop
-    \exp_args:NNx \int_set:Nn \l_@@_head_char_int
+    \@@_exp_args:NNx \int_set:Nn \l_@@_head_char_int
       { ` \@@_token_to_char:N \l_@@_head_token }
   }
 \cs_new_protected:Npn \@@_set_cmd_aux_char:w #1 ~ #2 \q_stop
@@ -2470,8 +2537,9 @@
         \gtl_if_head_is_N_type:NT #1
           {
             \tl_set:Nx \l_@@_input_tmpa_tl { \gtl_left_tl:N #1 }
-            \exp_last_unbraced:Nx \@@_input_split_auxii:N
-              { \tl_head:N \l_@@_input_tmpa_tl }
+            \@@_exp_args:NNx \use:nn
+                \@@_input_split_auxii:N
+                { \tl_head:N \l_@@_input_tmpa_tl }
           }
       }
   }
@@ -2546,7 +2614,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_input_merge:}
+% \begin{macro}{\@@_input_merge:}
 %   Merge the top two levels of input.  This requires, but does not
 %   check, that \cs{g_@@_input_int} is at least~$2$.
 %    \begin{macrocode}
@@ -2564,7 +2632,7 @@
 % \end{macro}
 %
 % \begin{macro}[int,TF]{\@@_input_gpop_item:N}
-% \begin{macro}[aux]{\@@_input_gpop_item_aux:NN}
+% \begin{macro}{\@@_input_gpop_item_aux:NN}
 %   If there is no input, we cannot pop an item.  Othewise, try to pop
 %   from the top of the input stack.  If this succeeds, or if this
 %   failed and the top of stack has extra end-group characters, or if
@@ -2656,7 +2724,9 @@
     \gtl_gclear_new:c { g_@@_input_ \int_use:N \g_@@_input_int _gtl }
     \gtl_gset:cn { g_@@_input_ \int_use:N \g_@@_input_int _gtl }
   }
-\cs_generate_variant:Nn \@@_back_input:n { x , V , o }
+\cs_generate_variant:Nn \@@_back_input:n { V , o }
+\cs_new_protected:Npn \@@_back_input:x
+  { \@@_exp_args:Nx \@@_back_input:n }
 %    \end{macrocode}
 % \end{macro}
 %
@@ -2723,10 +2793,11 @@
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_insert_group_begin_error:
   {
-    \msg_error:nn { unravel  } { missing-lbrace }
+    \tl_set_eq:NN \l_@@_tmpa_tl \l_@@_head_tl
     \@@_back_input:
     \gtl_set_eq:NN \l_@@_head_gtl \c_group_begin_gtl
     \@@_back_input:
+    \@@_tex_error:nV { missing-lbrace } \l_@@_tmpa_tl
     \@@_print_action:
   }
 %    \end{macrocode}
@@ -2738,7 +2809,7 @@
   {
     \@@_back_input:
     \@@_back_input:n { $ } % $
-    \msg_error:nn { unravel } { missing-dollar }
+    \@@_error:nnnnn { missing-dollar } { } { } { } { }
     \@@_print_action:
   }
 %    \end{macrocode}
@@ -2746,7 +2817,7 @@
 %
 % \subsubsection{Macro calls}
 %
-% \begin{macro}[aux,EXP]
+% \begin{macro}[EXP]
 %   {
 %     \@@_macro_prefix:N,
 %     \@@_macro_parameter:N,
@@ -2778,7 +2849,7 @@
 %
 % ^^A todo: redoc
 % \begin{macro}[int]{\@@_macro_call:}
-% \begin{macro}[aux]
+% \begin{macro}
 %   {
 %     \@@_macro_call_safe:,
 %     \@@_macro_call_quick:,
@@ -2826,8 +2897,8 @@
   }
 \cs_new_protected:Npn \@@_macro_call_quick_runaway:Nw #1#2 \q_stop
   {
-    \msg_error:nnxx { unravel } { runaway-macro-parameter }
-      { \tl_to_str:N \l_@@_head_tl } { \tl_to_str:n {#1} }
+    \@@_error:nxxxx { runaway-macro-parameter }
+      { \tl_to_str:N \l_@@_head_tl } { \tl_to_str:n {#1} } { } { }
   }
 %    \end{macrocode}
 % \end{macro}
@@ -2847,7 +2918,7 @@
     \bool_if:NT \g_@@_internal_debug_bool
       {
         \@@_set_cmd:
-        \iow_term:x { Exp:~\int_to_arabic:n { \l_@@_head_cmd_int } }
+        \@@_exp_args:Nx \iow_term:n { Exp:~\int_to_arabic:n { \l_@@_head_cmd_int } }
       }
     \token_if_macro:NTF \l_@@_head_token
       { \@@_macro_call: }
@@ -2871,11 +2942,11 @@
       {
         \cs_if_exist_use:cF
           { @@_expandable_ \int_use:N \l_@@_head_cmd_int : }
-          { \msg_error:nnx { unravel } { internal } { expandable } }
+          { \@@_error:nxxxx { internal } { expandable } { } { } { } }
       }
       {
-        \msg_error:nnx { unravel } { unknown-primitive }
-          { \l_@@_head_meaning_tl }
+        \@@_error:nxxxx { unknown-primitive }
+          { \l_@@_head_meaning_tl } { } { } { }
         \gtl_gput_right:NV \g_@@_output_gtl \l_@@_head_tl
         \@@_print_action:
       }
@@ -2921,7 +2992,7 @@
 % ^^A todo: implement |test_no_ligatures|?
 % ^^A todo: implement |get_tag_code|?
 %
-% \begin{macro}[aux]{\@@_get_x_non_blank:}
+% \begin{macro}{\@@_get_x_non_blank:}
 %   This function does not set the |cmd| and |char| integers.
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_get_x_non_blank:
@@ -2933,7 +3004,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_get_x_non_relax:}
+% \begin{macro}{\@@_get_x_non_relax:}
 %   This function does not set the |cmd| and |char| integers.
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_get_x_non_relax:
@@ -2949,7 +3020,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_skip_optional_space:}
+% \begin{macro}{\@@_skip_optional_space:}
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_skip_optional_space:
   {
@@ -2960,11 +3031,11 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_scan_optional_equals:}
+% \begin{macro}{\@@_scan_optional_equals:}
 %   See \TeX{}'s |scan_optional_equals|.  In all cases we forcefully
 %   insert an equal sign in the output, because this sign is required,
 %   as \cs{@@_scan_something_internal:n} leaves raw numbers in
-%   \cs{g_@@_prev_input_seq}.
+%   the previous-input sequence.
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_scan_optional_equals:
   {
@@ -2980,21 +3051,25 @@
 % \end{macro}
 %
 % \begin{macro}[int]{\@@_scan_left_brace:}
-%   The presence of \tn{relax} is allowed before a begin-group
-%   character.
+%   The presence of \tn{relax} is allowed before a begin-group token.
+%   If there is no begin-group token, insert one, produce an error, and
+%   scan that begin-group using \cs{@@_get_next:}.
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_scan_left_brace:
   {
     \@@_get_x_non_relax:
     \token_if_eq_catcode:NNF \l_@@_head_token \c_group_begin_token
-      { \@@_insert_group_begin_error: }
+      {
+        \@@_insert_group_begin_error:
+        \@@_get_next:
+      }
   }
 %    \end{macrocode}
 % \end{macro}
 %
 % \begin{macro}[int]{\@@_scan_keyword:n}
 % \begin{macro}[int,TF]{\@@_scan_keyword:n}
-% \begin{macro}[aux]
+% \begin{macro}
 %   {
 %     \@@_scan_keyword_loop:NNN,
 %     \@@_scan_keyword_test:NNTF,
@@ -3019,7 +3094,7 @@
 %   at the first character only; spaces become an empty string, but this
 %   works out because no keyword contains a space.  So, at each
 %   iteration, if the token is the correct non-active character, add it
-%   to \cs{g_@@_prev_input_seq} (as a generalized token list since
+%   to the previous-input sequence (as a generalized token list since
 %   keywords may match begin-group or end-group characters), and
 %   otherwise break with \cs{@@_scan_keyword_false:w}, unless we are
 %   still at the beginning of the keyword and the token is a space.
@@ -3040,7 +3115,7 @@
 \prg_new_protected_conditional:Npnn \@@_scan_keyword:n #1
   { T , F , TF }
   {
-    \seq_gput_right:NV \g_@@_prev_input_seq \c_empty_gtl
+    \@@_prev_input_gpush_gtl:
     \@@_scan_keyword_loop:NNN \c_true_bool
       #1 \q_recursion_tail \q_recursion_tail \q_recursion_stop
   }
@@ -3049,7 +3124,7 @@
     \quark_if_recursion_tail_stop_do:nn {#2}
       { \@@_scan_keyword_true: }
     \quark_if_recursion_tail_stop_do:nn {#3}
-      { \msg_error:nnx { unravel } { internal } { odd-keyword-length } }
+      { \@@_error:nxxxx { internal } { odd-keyword-length } { } { } { } }
     \@@_get_x_next:
     \@@_scan_keyword_test:NNTF #2#3
       {
@@ -3083,7 +3158,7 @@
   }
 \cs_new_protected_nopar:Npn \@@_scan_keyword_true:
   {
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_tmpb_gtl
+    \@@_prev_input_gpop_gtl:N \l_@@_tmpb_gtl
     \@@_prev_input:x { \gtl_to_str:N \l_@@_tmpb_gtl }
     \prg_return_true:
   }
@@ -3091,7 +3166,7 @@
     #1 \q_recursion_stop
   {
     \@@_back_input:
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_tmpb_gtl
+    \@@_prev_input_gpop_gtl:N \l_@@_tmpb_gtl
     \@@_back_input_gtl:N \l_@@_tmpb_gtl
     \prg_return_false:
   }
@@ -3100,7 +3175,7 @@
 % \end{macro}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_scan_to:}
+% \begin{macro}{\@@_scan_to:}
 %   Used when |to| is mandatory: after \tn{read} or \tn{readline} and
 %   after \tn{vsplit}.
 %    \begin{macrocode}
@@ -3108,14 +3183,14 @@
   {
     \@@_scan_keyword:nF { tToO }
       {
-        \msg_error:nn { unravel } { missing-to }
+        \@@_error:nnnnn { missing-to } { } { } { } { }
         \@@_prev_input:n { to }
       }
   }
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_scan_font_ident:}
+% \begin{macro}{\@@_scan_font_ident:}
 %   Find a font identifier.
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_scan_font_ident:
@@ -3139,7 +3214,7 @@
           }
       }
       {
-        \msg_error:nn { unravel } { missing-font-id }
+        \@@_error:nnnnn { missing-font-id } { } { } { } { }
         \@@_back_input:
         \@@_prev_input:n { \@@_nullfont: }
       }
@@ -3147,7 +3222,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_scan_font_int:}
+% \begin{macro}{\@@_scan_font_int:}
 %   Find operands for one of \tn{hyphenchar}'s friends (command code
 %   |assign_font_int=78|).
 %    \begin{macrocode}
@@ -3164,7 +3239,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_scan_font_dimen:}
+% \begin{macro}{\@@_scan_font_dimen:}
 %   Find operands for \tn{fontdimen}.
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_scan_font_dimen:
@@ -3175,10 +3250,9 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% ^^A todo: redoc
 % ^^A todo: rename this function because all other scan functions do get_x_next themselves.
-% \begin{macro}[aux]{\@@_scan_something_internal:n}
-% \begin{macro}[aux]{\@@_scan_something_aux:nwn}
+% \begin{macro}{\@@_scan_something_internal:n}
+% \begin{macro}{\@@_scan_something_aux:nwn}
 %   Receives an (explicit) \enquote{level} argument:
 %   \begin{itemize}
 %   \item |int_val=0| for integer values;
@@ -3186,33 +3260,40 @@
 %   \item |glue_val=2| for glue specifications;
 %   \item |mu_val=3| for math glue specifications;
 %   \item |ident_val=4| for font identifiers (this never happens);
-%   \item |tok_val=5| for token lists.
+%   \item |tok_val=5| for token lists (after |\the| or |\showthe|).
 %   \end{itemize}
 %   Scans something internal, and places its value, converted to the
 %   given level, to the right of the last item of
-%   \cs{g_@@_prev_input_seq}, then sets \cs{g_@@_val_level_int} to the
+%   the previous-input sequence, then sets \cs{g_@@_val_level_int} to the
 %   found level (level before conversion, so this may be higher than
-%   requested).  Get in one go the information about what level is
+%   requested).
+%
+%   From \cs{@@_thing_case:}, get the information about what level is
 %   produced by the given token once it has received all its operands
 %   (head of \cs{l_@@_tmpa_tl}), and about what to do to find those
-%   operands (tail of \cs{l_@@_tmpa_tl}).  If the first token is not
-%   between |min_internal=68| and |max_internal=89|, this step claims a
-%   level of~$8$.  If the level that will be produced is $4$ or~$5$, but
-%   the argument~|#1| is not, or if the level is~$8$ (exercise: check
-%   that the conditional indeed checks for this case, given that \eTeX{}
-%   rounds ``to nearest, ties away from zero''), then complain.
-%   Otherwise, fetch arguments if there are any: the scanning is
-%   performed after placing the current token in a new level of
-%   |prev_input| and telling the user about it.  Once done with this
-%   step, \cs{l_@@_head_tl} contains the tokens found.  Convert them to
-%   the wanted level with \cs{@@_thing_use_get:nnNN} (at this stage, \TeX{}
-%   may complain about a missing number or incompatible glue units), and
-%   place the result in |prev_input|.  Finally, tell the user the tokens
-%   that have been found and their value (and, if there was a single
-%   token, its meaning).  Use |=>| rather than |=| because the value
-%   displayed is the value used, not the actual value (this matters in
-%   constructions such as |\parindent=\parskip| where a skip or a dimen
-%   is downgraded to a dimen or an int).
+%   operands (tail of \cs{l_@@_tmpa_tl}).  If the first token may not
+%   appear after \tn{the} at all, \cs{@@_thing_case:} gives level~$8$.
+%
+%   If the argument (|#3|~in the auxiliary) is $<4$ but the level that
+%   will be produced (|#1|~in the auxiliary) is $\geq 4$ (that is,
+%   $4$, $5$, or $8$) complain about a missing number and insert a
+%   zero dimension, to get exactly \TeX{}'s error recovery.  If the
+%   level produced is~$8$, complain that |\the| cannot do this.
+%
+%   Otherwise, scan the arguments (in a new input level).  If both the
+%   argument and the level produced are $<4$, then get the value with
+%   \cs{@@_thing_use_get:nnNN} which downgrades from glue to dimension
+%   to integer and produces the |incompatible-units| error if needed.
+%   The only remaining case is that the argument is $5$ (since $4$ is
+%   never used) and the level produced is that or less: then the value
+%   found is used with \cs{@@_the:w}.
+%
+%   Finally, tell the user the tokens that have been found (if there
+%   was a single token, its meaning as well) and their value.  Use |=>|
+%   rather than |=| because the value displayed is the value used, not
+%   the actual value (this matters in constructions such as
+%   |\parindent=\parskip| where a skip or a dimen is downgraded to a
+%   dimen or an int, or when there was an error).
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_scan_something_internal:n #1
   {
@@ -3224,35 +3305,49 @@
   }
 \cs_new_protected:Npn \@@_scan_something_aux:nwn #1#2 \q_stop #3
   {
-    \int_compare:nNnTF
-      { ( #1 + \c_two ) / \c_four } > { ( #3 + \c_two ) / \c_four }
+    \int_compare:nT { #3 < 4 <= #1 }
       {
         \@@_back_input:
-        \msg_error:nn { unravel } { missing-something }
-        \tl_clear:N \l_@@_tmpa_tl
+        \@@_tex_error:nV { missing-number } \l_@@_head_tl
+        \@@_thing_use_get:nnNN { 1 } {#3} \c_zero_dim \l_@@_tmpa_tl
+        \@@_scan_something_internal_auxii:Vn \l_@@_tmpa_tl { 1 }
+        \@@_break:w
       }
+    \int_compare:nNnT {#1} = { 8 }
       {
-        \tl_if_empty:nF {#2}
-          {
-            \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
-            \@@_print_action:
-            #2
-            \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
-          }
-        \@@_thing_use_get:nnNN {#1} {#3} \l_@@_head_tl \l_@@_tmpa_tl
+        \@@_tex_error:nV { the-cannot } \l_@@_head_tl
+        \@@_scan_something_internal_auxii:Vn \c_zero { 0 }
+        \@@_break:w
       }
-    \@@_prev_input_silent:V \l_@@_tmpa_tl
+    \tl_if_empty:nF {#2}
+      {
+        \@@_prev_input_gpush:N \l_@@_head_tl
+        \@@_print_action:
+        #2
+        \@@_prev_input_gpop:N \l_@@_head_tl
+      }
+    \int_compare:nNnTF {#3} < { 4 }
+      { \@@_thing_use_get:nnNN {#1} {#3} \l_@@_head_tl \l_@@_tmpa_tl }
+      { \tl_set:Nx \l_@@_tmpa_tl { \@@_the:w \l_@@_head_tl } }
+    \@@_scan_something_internal_auxii:Vn \l_@@_tmpa_tl {#1}
+    \@@_break_point:
+    \int_compare:nNnT {#3} < { 4 } { \@@_print_action: }
+  }
+\cs_new_protected:Npn \@@_scan_something_internal_auxii:nn #1#2
+  {
+    \@@_prev_input_silent:n {#1}
     \@@_set_action_text:
     \@@_set_action_text:x
-      { \g_@@_action_text_str \use:n { ~ => ~ } \tl_to_str:N \l_@@_tmpa_tl }
-    \int_compare:nNnF {#3} > { 3 } { \@@_print_action: }
-    \int_gset:Nn \g_@@_val_level_int {#1}
+      { \g_@@_action_text_str \use:n { ~ => ~ } \tl_to_str:n {#1} }
+    \int_gset:Nn \g_@@_val_level_int {#2}
   }
+\cs_generate_variant:Nn \@@_scan_something_internal_auxii:nn { V }
 %    \end{macrocode}
 % \end{macro}
 % \end{macro}
 %
-% \begin{macro}[aux, EXP]
+% ^^A todo: \the can be followed by ignore_spaces
+% \begin{macro}[EXP]
 %   {\@@_thing_case:, \@@_thing_last_item:, \@@_thing_register:}
 %   This expands to a digit (the level generated by whatever token is
 %   the current |head|), followed by some code to fetch necessary
@@ -3290,6 +3385,8 @@
         { 87 } { 4                         } % set_font
         { 88 } { 4                         } % def_font
         { 89 } { \@@_thing_register:       } % register
+        {101 } { 4                         } % letterspace_font
+        {102 } { 4                         } % pdf_copy_font
       }
       { 8 }
   }
@@ -3347,7 +3444,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_scan_toks_register:}
+% \begin{macro}{\@@_scan_toks_register:}
 %   A case where getting operands is not completely trivial.
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_scan_toks_register:
@@ -3358,12 +3455,10 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_thing_use_get:nnNN}
-%   Given a level found~|#1| and a target level~|#2| (with |#1| in
-%   $[0,3]$ or |#1| and |#2| in $[4,5]$), turn the token list |#3| into
-%   the desired level, and store the result in |#4|.  This step may
-%   trigger \TeX{} errors, which should precisely match the expected
-%   ones.
+% \begin{macro}{\@@_thing_use_get:nnNN}
+%   Given a level found~|#1| and a target level~|#2| (both in $[0,3]$),
+%   turn the token list |#3| into the desired level or less, and store
+%   the result in |#4|.
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_thing_use_get:nnNN #1#2#3#4
   {
@@ -3383,27 +3478,23 @@
           }
       }
       {
-        \int_compare:nNnTF {#2} = { 3 }
+        \int_case:nnF {#1}
           {
-            \int_case:nnF {#1}
-              {
-                { 0 } { \tl_set:Nx #4 { \int_eval:n {#3} } }
-                { 3 } { \tl_set:Nx #4 { \muskip_eval:n {#3} } }
-              }
-              {
-                \@@_tex_error:nV { incompatible-units } #3
-                \tl_set:Nx #4 { \muskip_eval:n { \etex_gluetomu:D #3 } }
-              }
+            { 0 } { \tl_set:Nx #4 { \int_eval:n {#3} } }
+            { 3 } { \tl_set:Nx #4 { \muskip_eval:n {#3} } }
+          }
+          {
+            \@@_tex_error:nV { incompatible-units } #3
+            \tl_set:Nx #4 { \muskip_eval:n { \etex_gluetomu:D #3 } }
           }
-          { \tl_set:Nx #4 { \@@_the:w #3 } }
       }
   }
 %    \end{macrocode}
 % \end{macro}
 %
 % ^^A todo: comment
-% \begin{macro}[aux]{\@@_scan_expr:N}
-% \begin{macro}[aux]{\@@_scan_expr_aux:NN, \@@_scan_factor:N}
+% \begin{macro}{\@@_scan_expr:N}
+% \begin{macro}{\@@_scan_expr_aux:NN, \@@_scan_factor:N}
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_scan_expr:N #1
   { \@@_scan_expr_aux:NN #1 \c_false_bool }
@@ -3452,7 +3543,7 @@
       {
         \bool_if:NTF #2
           {
-            \msg_error:nn { unravel } { missing-rparen }
+            \@@_error:nnnnn { missing-rparen } { } { } { } { }
             \@@_back_input:
             \@@_prev_input:V \c_@@_rp_tl
           }
@@ -3478,7 +3569,7 @@
 % \end{macro}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_scan_signs:}
+% \begin{macro}{\@@_scan_signs:}
 %   Skips blanks, scans signs, and places them to the right of the last
 %   item of \cs{@@_prev_input:n}.
 %    \begin{macrocode}
@@ -3501,8 +3592,8 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_scan_int:}
-% \begin{macro}[aux]
+% \begin{macro}{\@@_scan_int:}
+% \begin{macro}
 %   {\@@_scan_int_char:, \@@_scan_int_lq:, \@@_scan_int_explicit:n}
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_scan_int:
@@ -3522,16 +3613,16 @@
           {
             \@@_prev_input:V \l_@@_head_tl
             \@@_get_x_next:
-            \@@_scan_int_explicit:n { ' }
+            \@@_scan_int_explicit:Nn \c_false_bool { ' }
           }
         \c_@@_dq_tl
           {
             \@@_prev_input:V \l_@@_head_tl
             \@@_get_x_next:
-            \@@_scan_int_explicit:n { " }
+            \@@_scan_int_explicit:Nn \c_false_bool { " }
           }
       }
-      { \@@_scan_int_explicit:n { } }
+      { \@@_scan_int_explicit:Nn \c_false_bool { } }
   }
 \cs_new_protected_nopar:Npn \@@_scan_int_lq:
   {
@@ -3548,10 +3639,10 @@
       { ` \gtl_to_str:N \l_@@_head_gtl = \l_@@_tmpa_tl }
     \@@_skip_optional_space:
   }
-\cs_new_protected:Npn \@@_scan_int_explicit:n #1
+\cs_new_protected:Npn \@@_scan_int_explicit:Nn #1#2
   {
     \if_int_compare:w \c_one
-        < #1 1 \exp_after:wN \exp_not:N \l_@@_head_tl \exp_stop_f:
+        < #2 1 \exp_after:wN \exp_not:N \l_@@_head_tl \exp_stop_f:
       \exp_after:wN \use_i:nn
     \else:
       \exp_after:wN \use_ii:nn
@@ -3559,11 +3650,16 @@
     {
       \@@_prev_input:V \l_@@_head_tl
       \@@_get_x_next:
-      \@@_scan_int_explicit:n {#1}
+      \@@_scan_int_explicit:Nn \c_true_bool {#2}
     }
     {
       \token_if_eq_catcode:NNF \l_@@_head_token \c_space_token
         { \@@_back_input: }
+      \bool_if:NF #1
+        {
+          \@@_tex_error:nV { missing-number } \l_@@_head_tl
+          \@@_prev_input:n { 0 }
+        }
     }
   }
 %    \end{macrocode}
@@ -3585,12 +3681,12 @@
 %   \TeX{}'s own |scan_dimen| procedure, in which |mu| is |bool(#1=3)|
 %   and |inf| is |#2|.  The third argument of this procedure is omitted
 %   here, as the corresponding shortcut is provided as a separate
-%   function, \cs{@@_scan_dimen_unit:nN}.
+%   function, \cs{@@_scan_dim_unit:nN}.
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_scan_dimen:nN #1#2
   {
     \@@_scan_signs:
-    \seq_gput_right:Nn \g_@@_prev_input_seq { }
+    \@@_prev_input_gpush:
     \@@_set_cmd:
     \@@_cmd_if_internal:TF
       {
@@ -3601,7 +3697,7 @@
           { \@@_scan_dim_unit:nN {#1} #2 }
       }
       { \@@_scan_dimen_char:nN {#1} #2 }
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpop:N \l_@@_head_tl
     \@@_prev_input_silent:V \l_@@_head_tl
   }
 \cs_new_protected:Npn \@@_scan_dimen_char:nN #1#2
@@ -3648,17 +3744,18 @@
     \@@_set_cmd:
     \@@_cmd_if_internal:TF
       {
-        \seq_gput_right:Nn \g_@@_prev_input_seq { }
+        \@@_prev_input_gpush:
         \@@_scan_something_internal:n {#1}
         \@@_prev_input_join_get:nN {#1} \l_@@_tmpa_tl
-        \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_tmpa_tl
+        \@@_prev_input_gpush:N \l_@@_tmpa_tl
         \@@_break:w
       }
       { \@@_back_input: }
     \int_compare:nNnT {#1} = { 3 }
       {
         \@@_scan_keyword:nT { mMuU } { \@@_break:w }
-        \msg_error:nn { unravel } { missing-mudim }
+        \@@_tex_error:nV { missing-mu } \l_@@_head_tl
+        \@@_prev_input:n { mu }
         \@@_break:w
       }
     \@@_scan_keyword:nT { eEmM } { \@@_break:w }
@@ -3677,6 +3774,8 @@
     \@@_scan_keyword:nT { nNdD } { \@@_break:w }
     \@@_scan_keyword:nT { nNcC } { \@@_break:w }
     \@@_scan_keyword:nT { sSpP } { \@@_break:w }
+    \@@_tex_error:nV { missing-pt } \l_@@_head_tl
+    \@@_prev_input:n { pt }
     \@@_break_point:
   }
 \cs_new_protected_nopar:Npn \@@_scan_inf_unit_loop:
@@ -3713,24 +3812,24 @@
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_scan_glue:n #1
   {
-    \seq_gput_right:Nn \g_@@_prev_input_seq { }
+    \@@_prev_input_gpush:
     \@@_scan_signs:
-    \seq_gput_right:Nn \g_@@_prev_input_seq { }
+    \@@_prev_input_gpush:
     \@@_set_cmd:
     \@@_cmd_if_internal:TF
       {
         \@@_scan_something_internal:n {#1}
         \int_case:nnF \g_@@_val_level_int
           {
-            { 0 } { \@@_scan_dimen:nN {#1} \c_false_bool }
+            { 0 } { \@@_scan_dim_unit:nN {#1} \c_false_bool }
             { 1 } { }
           }
           { \@@_break:w }
       }
       { \@@_back_input: \@@_scan_dimen:nN {#1} \c_false_bool }
     \@@_prev_input_join_get:nN {#1} \l_@@_tmpa_tl
-    \seq_gput_right:Nn \g_@@_prev_input_seq { }
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_tmpa_tl
+    \@@_prev_input_gpush:
+    \@@_prev_input_gpush:N \l_@@_tmpa_tl
     \@@_scan_keyword:nT { pPlLuUsS }
       { \@@_scan_dimen:nN {#1} \c_true_bool }
     \@@_scan_keyword:nT { mMiInNuUsS }
@@ -3784,7 +3883,7 @@
       { \@@_get_next: }
     \@@_gtl_if_head_is_definable:NF \l_@@_head_gtl
       {
-        \msg_error:nn { unravel } { missing-cs }
+        \@@_error:nnnnn { missing-cs } { } { } { } { }
         \@@_back_input:
         \tl_set:Nn \l_@@_head_tl { \@@_inaccessible:w }
       }
@@ -3795,13 +3894,13 @@
 % \end{macro}
 %
 % ^^A todo: hash doubling is wrong
-% \begin{macro}[aux]{\@@_scan_toks_to_str:}
+% \begin{macro}{\@@_scan_toks_to_str:}
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_scan_toks_to_str:
   {
-    \seq_gput_right:Nn \g_@@_prev_input_seq { }
+    \@@_prev_input_gpush:
     \@@_scan_toks:NN \c_false_bool \c_true_bool
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_tmpa_tl
+    \@@_prev_input_gpop:N \l_@@_tmpa_tl
     \@@_prev_input_silent:x
       { { \exp_after:wN \tl_to_str:n \l_@@_tmpa_tl } }
   }
@@ -3809,7 +3908,7 @@
 % \end{macro}
 %
 % ^^A todo: provide separate functions for defs (avoid hash doubling).
-% \begin{macro}[aux]{\@@_scan_toks:NN}
+% \begin{macro}{\@@_scan_toks:NN}
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_scan_toks:NN #1#2
   {
@@ -3822,8 +3921,8 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_scan_param:}
-% \begin{macro}[aux]{\@@_scan_param_aux:}
+% \begin{macro}{\@@_scan_param:}
+% \begin{macro}{\@@_scan_param_aux:}
 %   Collect the parameter text into \cs{l_@@_tmpa_tl}, and when seeing
 %   either a begin-group or an end-group character, put it back into the
 %   input, stop looping, and put what we collected into
@@ -3848,14 +3947,14 @@
 % \end{macro}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_scan_group_n:N}
+% \begin{macro}{\@@_scan_group_n:N}
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_scan_group_n:N #1
   {
     \@@_back_input:
     \@@_input_gpop_item:NF \l_@@_head_tl
       {
-        \msg_error:nn { unravel } { runaway-text }
+        \@@_error:nnnnn { runaway-text } { } { } { } { }
         \@@_exit:w
       }
     \tl_set:Nx \l_@@_head_tl { { \exp_not:V \l_@@_head_tl } }
@@ -3866,7 +3965,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_scan_group_x:N}
+% \begin{macro}{\@@_scan_group_x:N}
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_scan_group_x:N #1
   {
@@ -3879,10 +3978,10 @@
         \@@_scan_group_xdef:n { 1 }
       }
       {
-        \seq_gput_right:NV \g_@@_prev_input_seq \c_empty_gtl
+        \@@_prev_input_gpush_gtl:
         \@@_prev_input_gtl:N \l_@@_head_gtl
         \@@_scan_group_x:n { 1 }
-        \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_tmpb_gtl
+        \@@_prev_input_gpop_gtl:N \l_@@_tmpb_gtl
         \@@_prev_input_silent:x
           { \gtl_left_tl:N \l_@@_tmpb_gtl }
       }
@@ -3890,7 +3989,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_scan_group_xdef:n}
+% \begin{macro}{\@@_scan_group_xdef:n}
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_scan_group_xdef:n #1
   {
@@ -3921,7 +4020,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_scan_group_x:n}
+% \begin{macro}{\@@_scan_group_x:n}
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_scan_group_x:n #1
   {
@@ -3942,7 +4041,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_get_token_x:N}
+% \begin{macro}{\@@_get_token_x:N}
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_get_token_x:N #1
   {
@@ -3960,7 +4059,7 @@
                 \@@_prev_input:V \l_@@_head_tl
               }
               {
-                \gtl_set:Nx \l_@@_tmpb_gtl { \l_@@_head_tl }
+                \@@_exp_args:NNx \gtl_set:Nn \l_@@_tmpb_gtl { \l_@@_head_tl }
                 \@@_prev_input_gtl:N \l_@@_tmpb_gtl
                 \@@_print_action:
               }
@@ -3974,7 +4073,7 @@
 %
 % ^^A todo: catch runaway expand/get_x_next. (get_next too, probably)
 %
-% \begin{macro}[aux]{\@@_scan_alt_rule:}
+% \begin{macro}{\@@_scan_alt_rule:}
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_scan_alt_rule:
   {
@@ -4001,7 +4100,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_scan_spec:}
+% \begin{macro}{\@@_scan_spec:}
 %   Some \TeX{} primitives accept the keywords |to| and |spread|,
 %   followed by a dimension.
 %    \begin{macrocode}
@@ -4019,9 +4118,9 @@
 %
 % \subsection{Working with boxes}
 %
-% \begin{macro}[aux]{\@@_do_box:N}
+% \begin{macro}{\@@_do_box:N}
 %   When this procedure is called, the last item in
-%   \cs{g_@@_prev_input_seq} is
+%   the previous-input sequence is
 %   \begin{itemize}
 %   \item empty if the box is meant to be put in the input stream,
 %   \item \tn{setbox}\meta{int} if it is meant to be stored somewhere,
@@ -4059,26 +4158,26 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_do_box_error:}
+% \begin{macro}{\@@_do_box_error:}
 %   Put the (non-|make_box|) command back into the input and complain.
 %   Then recover by throwing away the action (last item of
-%   \cs{g_@@_prev_input_seq}).  For some reason (this appears to be what
+%   the previous-input sequence).  For some reason (this appears to be what
 %   \TeX{} does), there is no need to remove the after assignment token
 %   here.
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_do_box_error:
   {
     \@@_back_input:
-    \msg_error:nn { unravel } { missing-box }
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_error:nnnnn { missing-box } { } { } { } { }
+    \@@_prev_input_gpop:N \l_@@_head_tl
     \@@_print_action:x { \tl_to_str:N \l_@@_head_tl }
   }
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_do_begin_box:N}
+% \begin{macro}{\@@_do_begin_box:N}
 %   We have just found a |make_box| command and placed it into the last
-%   item of \cs{g_@@_prev_input_seq}.  If it is ``simple''
+%   item of the previous-input sequence.  If it is ``simple''
 %   (\tn{box}\meta{int}, \tn{copy}\meta{int}, \tn{lastbox},
 %   \tn{vsplit}\meta{int} \texttt{to} \meta{dim}) then we grab its
 %   operands, then call \cs{@@_do_simple_box:N} to finish up.  If it is
@@ -4105,7 +4204,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_do_simple_box:N}
+% \begin{macro}{\@@_do_simple_box:N}
 %   For leaders, we need to fetch a glue.  In all cases, retrieve the
 %   box construction (such as |\raise3pt\vsplit7to5em|).  Finally, let
 %   \TeX{} run the code and print what we have done.
@@ -4116,7 +4215,7 @@
   {
     \bool_if:NTF #1 { \@@_do_leaders_fetch_skip: }
       {
-        \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpop:N \l_@@_head_tl
         \tl_if_head_eq_meaning:VNT \l_@@_head_tl \tex_shipout:D
           { \@@_prepare_mag: }
         \tl_use:N \l_@@_head_tl \scan_stop:
@@ -4127,7 +4226,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_do_leaders_fetch_skip:}
+% \begin{macro}{\@@_do_leaders_fetch_skip:}
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_do_leaders_fetch_skip:
   {
@@ -4136,14 +4235,14 @@
     \int_compare:nNnTF \l_@@_head_cmd_int
       = { \@@_tex_use:n { \mode_if_vertical:TF { vskip } { hskip } } }
       {
-        \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_tmpa_tl
+        \@@_prev_input_gpop:N \l_@@_tmpa_tl
         \tl_put_left:NV \l_@@_head_tl \l_@@_tmpa_tl
         \@@_do_append_glue:
       }
       {
         \@@_back_input:
-        \msg_error:nn { unravel } { improper-leaders }
-        \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_error:nnnnn { improper-leaders } { } { } { } { }
+        \@@_prev_input_gpop:N \l_@@_head_tl
         \@@_print_action:x { \tl_to_str:N \l_@@_head_tl }
       }
   }
@@ -4151,8 +4250,8 @@
 % \end{macro}
 %
 % ^^A todo: less intrusive hooking into \everyhbox/\everyvbox?
-% \begin{macro}[aux]{\@@_do_box_explicit:N}
-%   At this point, the last item in \cs{g_@@_prev_input_seq} is
+% \begin{macro}{\@@_do_box_explicit:N}
+%   At this point, the last item in the previous-input sequence is
 %   typically |\setbox0\hbox| or |\raise 3pt\hbox|.  Scan for keywords
 %   |to| and |spread| and a left brace.  Install a hook in
 %   \tn{everyhbox} or \tn{everyvbox} (whichever \TeX{} is going to
@@ -4160,7 +4259,7 @@
 %   the current box into \cs{l_@@_head_tl} in order to print it, then
 %   let \TeX{} perform the box operation (here we need to provide the
 %   begin-group token, as it was scanned but not placed in
-%   \cs{g_@@_prev_input_seq}).  \TeX{} inserts \tn{everyhbox} or
+%   the previous-input sequence).  \TeX{} inserts \tn{everyhbox} or
 %   \tn{everyvbox} just after the begin-group token, and the hook we did
 %   is such that all that material is collected and put into the input
 %   that we will study.
@@ -4175,10 +4274,10 @@
       { \@@_box_hook:N \tex_everyvbox:D }
       % ^^A todo: TeX calls |normal_paragraph| here.
     \@@_scan_spec:
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpop:N \l_@@_head_tl
     \@@_set_action_text:x
       { \tl_to_str:N \l_@@_head_tl \iow_char:N \{ }
-    \seq_push:Nx \l_@@_leaders_box_seq
+    \seq_push:Nf \l_@@_leaders_box_seq
       { \bool_if:NTF #1 { \mode_if_vertical:TF { v } { h } } { Z } }
     \gtl_gput_right:NV \g_@@_output_gtl \l_@@_head_tl
     \gtl_gconcat:NNN \g_@@_output_gtl
@@ -4189,8 +4288,8 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_box_hook:N}
-% \begin{macro}[aux]{\@@_box_hook:w, \@@_box_hook_end:}
+% \begin{macro}{\@@_box_hook:N}
+% \begin{macro}{\@@_box_hook:w, \@@_box_hook_end:}
 %   Used to capture the contents of an \tn{everyhbox} or similar,
 %   without altering \tn{everyhbox} too much (just add one token at the
 %   start).  The various \texttt{o}-expansions remove
@@ -4202,7 +4301,7 @@
     \str_if_eq_x:nnF
       { \tl_head:N \l_@@_tmpa_tl } { \exp_not:N \@@_box_hook:w }
       {
-        \exp_args:Nx #1
+        \@@_exp_args:Nx #1
           {
             \exp_not:n { \@@_box_hook:w \prg_do_nothing: }
             \exp_not:V #1
@@ -4226,7 +4325,7 @@
 % \end{macro}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_do_leaders_rule:}
+% \begin{macro}{\@@_do_leaders_rule:}
 %   After finding a |vrule| or |hrule| command and looking for |depth|,
 %   |heigh| and |width| keywords, we are in the same situation as after
 %   finding a box.  Fetch the required skip accordingly.
@@ -4259,7 +4358,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_char:n, \@@_char:V, \@@_char:x}
+% \begin{macro}{\@@_char:n, \@@_char:V, \@@_char:x}
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_char:n #1
   {
@@ -4273,28 +4372,32 @@
     \gtl_gput_right:NV \g_@@_output_gtl \l_@@_tmpa_tl
     \@@_print_action:x { \tl_to_str:N \l_@@_tmpa_tl }
   }
-\cs_generate_variant:Nn \@@_char:n { V , x }
+\cs_generate_variant:Nn \@@_char:n { V }
+\cs_new_protected:Npn \@@_char:x
+  { \@@_exp_args:Nx \@@_char:n }
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]
+% \begin{macro}
 %   {\@@_char_in_mmode:n, \@@_char_in_mmode:V, \@@_char_in_mmode:x}
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_char_in_mmode:n #1
   {
     \int_compare:nNnTF { \tex_mathcode:D #1 } = { "8000 }
       { % math active
-        \gtl_set:Nx \l_@@_head_gtl
+        \@@_exp_args:NNx \gtl_set:Nn \l_@@_head_gtl
           { \char_generate:nn {#1} { 12 } }
         \@@_back_input:
       }
       { \@@_char:n {#1} }
   }
-\cs_generate_variant:Nn \@@_char_in_mmode:n { V , x }
+\cs_generate_variant:Nn \@@_char_in_mmode:n { V }
+\cs_new_protected:Npn \@@_char_in_mmode:x
+  { \@@_exp_args:Nx \@@_char_in_mmode:n }
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_mathchar:n, \@@_mathchar:x}
+% \begin{macro}{\@@_mathchar:n, \@@_mathchar:x}
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_mathchar:n #1
   {
@@ -4304,7 +4407,8 @@
     \gtl_gput_right:NV \g_@@_output_gtl \l_@@_tmpa_tl
     \@@_print_action:x { \tl_to_str:N \l_@@_tmpa_tl }
   }
-\cs_generate_variant:Nn \@@_mathchar:n { x }
+\cs_new_protected:Npn \@@_mathchar:x
+  { \@@_exp_args:Nx \@@_mathchar:n }
 %    \end{macrocode}
 % \end{macro}
 %
@@ -4367,7 +4471,7 @@
 % ^^A todo: implement \@@_end_disc_group:
 % ^^A todo: implement \@@_end_math_choice_group:
 %
-% \begin{macro}[aux]{\@@_handle_right_brace:}
+% \begin{macro}{\@@_handle_right_brace:}
 %   When an end-group character is sensed, the result depends on the
 %   current group type.
 %    \begin{macrocode}
@@ -4403,7 +4507,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_end_simple_group:}
+% \begin{macro}{\@@_end_simple_group:}
 %   This command is used to simply end a group, when there are no
 %   specific operations to perform.
 %    \begin{macrocode}
@@ -4415,7 +4519,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_end_box_group:}
+% \begin{macro}{\@@_end_box_group:}
 %   The end of an explicit box (generated by \tn{vtop}, \tn{vbox}, or
 %   \tn{hbox}) can either be simple, or can mean that we need to find a
 %   skip for a \tn{leaders}/\tn{cleaders}/\tn{xleaders} construction.
@@ -4448,14 +4552,14 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_off_save:}
+% \begin{macro}{\@@_off_save:}
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_off_save:
   {
     \int_compare:nNnTF \@@_currentgrouptype: = { 0 }
       { % bottom-level
-        \msg_error:nnx { unravel } { extra-close }
-          { \token_to_meaning:N \l_@@_head_token }
+        \@@_error:nxxxx { extra-close }
+          { \token_to_meaning:N \l_@@_head_token } { } { } { }
       }
       {
         \@@_back_input:
@@ -4470,8 +4574,8 @@
           }
           { \gtl_set_eq:NN \l_@@_head_gtl \c_group_end_gtl }
         \@@_back_input:
-        \msg_error:nnx { unravel } { off-save }
-          { \gtl_to_str:N \l_@@_head_gtl }
+        \@@_error:nxxxx { off-save }
+          { \gtl_to_str:N \l_@@_head_gtl } { } { } { }
       }
   }
 %    \end{macrocode}
@@ -4503,7 +4607,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_head_for_vmode:}
+% \begin{macro}{\@@_head_for_vmode:}
 %   See \TeX{}'s |head_for_vmode|.
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_head_for_vmode:
@@ -4512,7 +4616,7 @@
       {
         \token_if_eq_meaning:NNTF \l_@@_head_token \tex_hrule:D
           {
-            \msg_error:nn { unravel } { hrule-bad-mode }
+            \@@_error:nnnnn { hrule-bad-mode } { } { } { } { }
             \@@_print_action:
           }
           { \@@_off_save: }
@@ -4527,7 +4631,7 @@
 % \end{macro}
 %
 % ^^A todo: should push_math be provided?
-% \begin{macro}[aux]{\@@_goto_inner_math:}
+% \begin{macro}{\@@_goto_inner_math:}
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_goto_inner_math:
   {
@@ -4538,7 +4642,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_goto_display_math:}
+% \begin{macro}{\@@_goto_display_math:}
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_goto_display_math:
   {
@@ -4549,7 +4653,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_after_math:}
+% \begin{macro}{\@@_after_math:}
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_after_math:
   {
@@ -4567,7 +4671,7 @@
           {
             \@@_back_input:
             \tl_set:Nn \l_@@_head_tl { $ } % $
-            \msg_error:nn { unravel } { missing-dollar }
+            \@@_error:nnnnn { missing-dollar } { } { } { } { }
           }
         \gtl_gput_right:NV \g_@@_output_gtl \l_@@_head_tl
         \@@_back_input_gtl:N \l_@@_after_group_gtl
@@ -4588,10 +4692,10 @@
   {
     \@@_set_action_text:
     \bool_if:NT \g_@@_internal_debug_bool
-      { \iow_term:x { Cmd:~\int_to_arabic:n { \l_@@_head_cmd_int } } }
+      { \@@_exp_args:Nx \iow_term:n { Cmd:~\int_to_arabic:n { \l_@@_head_cmd_int } } }
     \cs_if_exist_use:cF
       { @@_cmd_ \int_use:N \l_@@_head_cmd_int : }
-      { \msg_error:nnx { unravel } { internal } { unknown-command } }
+      { \@@_error:nxxxx { internal } { unknown-command } { } { } { } }
   }
 %    \end{macrocode}
 % \end{macro}
@@ -4706,7 +4810,7 @@
   { \@@_mode_math:n { \@@_sub_sup: } }
 \cs_new_protected_nopar:Npn \@@_sub_sup:
   {
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpush:N \l_@@_head_tl
     \@@_print_action:
     \@@_get_x_non_relax:
     \@@_set_cmd:
@@ -4734,14 +4838,14 @@
           { \@@_prev_input:V \l_@@_head_tl \@@_scan_int: }
       }
       {
-        \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpop:N \l_@@_head_tl
         \gtl_gput_right:NV \g_@@_output_gtl \l_@@_head_tl
         \tl_use:N \l_@@_head_tl \scan_stop:
       }
       {
         \@@_back_input:
         \@@_scan_left_brace:
-        \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpop:N \l_@@_head_tl
         \gtl_gput_right:NV \g_@@_output_gtl \l_@@_head_tl
         \gtl_gconcat:NNN \g_@@_output_gtl
           \g_@@_output_gtl \c_group_begin_gtl
@@ -4753,7 +4857,7 @@
 %
 %    \begin{macrocode}
 \@@_new_tex_cmd:nn { endv }                                 % 9
-  { \msg_error:nn { unravel } { not-implemented } { alignments } }
+  { \@@_not_implemented:n { alignments } }
 %    \end{macrocode}
 %
 % Blank spaces are ignored in vertical and math modes in the same way as
@@ -4841,10 +4945,10 @@
   {
     \@@_mode_math:n
       {
-        \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpush:N \l_@@_head_tl
         \@@_print_action:
         \@@_scan_int:
-        \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpop:N \l_@@_head_tl
         \tl_use:N \l_@@_head_tl \scan_stop:
         \@@_print_action:x { \tl_to_str:N \l_@@_head_tl }
       }
@@ -4887,10 +4991,10 @@
   {
     \@@_mode_non_vertical:n
       {
-        \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpush:N \l_@@_head_tl
         \@@_print_action:
         \@@_scan_int:
-        \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpop:N \l_@@_head_tl
         \mode_if_math:TF
           { \@@_char_in_mmode:x { \tl_tail:N \l_@@_head_tl } }
           { \@@_char:x { \tl_tail:N \l_@@_head_tl } }
@@ -4906,10 +5010,10 @@
   {
     \@@_mode_math:n
       {
-        \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpush:N \l_@@_head_tl
         \@@_print_action:
         \@@_scan_int:
-        \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpop:N \l_@@_head_tl
         \@@_mathchar:x { \tl_tail:N \l_@@_head_tl }
       }
   }
@@ -4918,14 +5022,14 @@
 %    \begin{macrocode}
 \@@_new_tex_cmd:nn { mark }                                 % 18
   {
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpush:N \l_@@_head_tl
     \@@_print_action:
     \int_compare:nNnF \l_@@_head_char_int = \c_zero
       { \@@_scan_int: }
-    \seq_gput_right:Nn \g_@@_prev_input_seq { }
+    \@@_prev_input_gpush:
     \@@_scan_toks:NN \c_false_bool \c_true_bool
-    \seq_gpop_right:Nn \g_@@_prev_input_seq \l_@@_tmpa_tl
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpop:N \l_@@_tmpa_tl
+    \@@_prev_input_gpop:N \l_@@_head_tl
     \@@_print_action:x
       { \tl_to_str:N \l_@@_head_tl \tl_to_str:N \l_@@_tmpa_tl }
     \tl_put_right:Nx \l_@@_head_tl
@@ -4940,28 +5044,28 @@
 % Those with operands print first, then scan their operands, then are
 % sent to \TeX{}.  The case of \tn{show} is a bit special, as its
 % operand is a single token, which cannot easily be put into the
-% \cs{g_@@_prev_input_seq} in general.  Since no expansion can occur,
+% the previous-input sequence in general.  Since no expansion can occur,
 % simply grab the token and show it.
 %    \begin{macrocode}
 \@@_new_tex_cmd:nn { xray }                                 % 19
   {
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpush:N \l_@@_head_tl
     \@@_print_action:
     \int_case:nnF \l_@@_head_char_int
       {
         { 0 }
           { % show
             \@@_get_next:
-            \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_tmpa_tl
+            \@@_prev_input_gpop:N \l_@@_tmpa_tl
             \gtl_head_do:NN \l_@@_head_gtl \l_@@_tmpa_tl
           }
         { 2 }
           { % showthe
             \@@_get_x_next:
             \@@_scan_something_internal:n { 5 }
-            \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
-            \exp_args:Nx \etex_showtokens:D
-              { \tl_tail:N \l_@@_head_tl }
+            \@@_prev_input_gpop:N \l_@@_head_tl
+            \@@_exp_args:Nx \use:n
+              { \etex_showtokens:D { \tl_tail:N \l_@@_head_tl } }
           }
       }
       { % no operand for showlists, showgroups, showifs
@@ -4969,7 +5073,7 @@
           { \@@_scan_int: }
         \int_compare:nNnT \l_@@_head_char_int = \c_five % showtokens
           { \@@_scan_toks:NN \c_false_bool \c_false_bool }
-        \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpop:N \l_@@_head_tl
         \tl_use:N \l_@@_head_tl \scan_stop:
       }
   }
@@ -4980,7 +5084,7 @@
 %    \begin{macrocode}
 \@@_new_tex_cmd:nn { make_box }                             % 20
   {
-    \seq_gput_right:Nn \g_@@_prev_input_seq { }
+    \@@_prev_input_gpush:
     \@@_back_input:
     \@@_do_box:N \c_false_bool
   }
@@ -4988,13 +5092,13 @@
 %
 % ^^A todo: implement \@@_forbidden_case:
 %
-% \begin{macro}[aux]{\@@_do_move:}
+% \begin{macro}{\@@_do_move:}
 %   Scan a dimension and a box, and perform the shift, printing the
 %   appropriate action.
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_do_move:
   {
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpush:N \l_@@_head_tl
     \@@_print_action:
     \@@_scan_normal_dimen:
     \@@_do_box:N \c_false_bool
@@ -5020,14 +5124,14 @@
   }
 %    \end{macrocode}
 %
-% \begin{macro}[aux]{\@@_do_unpackage:}
+% \begin{macro}{\@@_do_unpackage:}
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_do_unpackage:
   {
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpush:N \l_@@_head_tl
     \@@_print_action:
     \@@_scan_int:
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpop:N \l_@@_head_tl
     \tl_use:N \l_@@_head_tl \scan_stop:
     \@@_print_action:x { \tl_to_str:N \l_@@_head_tl }
   }
@@ -5063,7 +5167,7 @@
   { \l_@@_head_token \@@_print_action: }
 %    \end{macrocode}
 %
-% \begin{macro}[aux]{\@@_do_append_glue:}
+% \begin{macro}{\@@_do_append_glue:}
 %   For \tn{hfil}, \tn{hfill}, \tn{hss}, \tn{hfilneg} and their vertical
 %   analogs, simply call the primitive then print the action.  For
 %   \tn{hskip}, \tn{vskip} and \tn{mskip}, read a normal glue or a mu
@@ -5075,11 +5179,11 @@
     \int_compare:nNnTF \l_@@_head_char_int < { 4 }
       { \tl_use:N \l_@@_head_tl \@@_print_action: }
       {
-        \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpush:N \l_@@_head_tl
         \@@_print_action:
         \exp_args:Nf \@@_scan_glue:n
           { \int_eval:n { \l_@@_head_char_int - 2 } }
-        \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpop:N \l_@@_head_tl
         \tl_use:N \l_@@_head_tl \scan_stop:
         \@@_print_action:x { \tl_to_str:N \l_@@_head_tl }
       }
@@ -5107,18 +5211,18 @@
   { \@@_mode_math:n { \@@_do_append_glue: } }
 %    \end{macrocode}
 %
-% \begin{macro}[aux]{\@@_do_append_kern:}
+% \begin{macro}{\@@_do_append_kern:}
 %   See \cs{@@_do_append_glue:}.  This function is used for the
 %   primitives \tn{kern} and \tn{mkern} only.
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_do_append_kern:
   {
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpush:N \l_@@_head_tl
     \@@_print_action:
     \token_if_eq_meaning:NNTF \l_@@_head_token \tex_kern:D
       { \@@_scan_dimen:nN { 2 } \c_false_bool }
       { \@@_scan_dimen:nN { 3 } \c_false_bool }
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpop:N \l_@@_head_tl
     \tl_use:N \l_@@_head_tl \scan_stop:
     \@@_print_action:x { \tl_to_str:N \l_@@_head_tl }
   }
@@ -5142,7 +5246,7 @@
 %    \begin{macrocode}
 \@@_new_tex_cmd:nn { leader_ship }                          % 31
   {
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpush:N \l_@@_head_tl
     \@@_print_action:
     \@@_do_box:N \c_true_bool
   }
@@ -5173,11 +5277,11 @@
 %
 %    \begin{macrocode}
 \@@_new_tex_cmd:nn { halign }                               % 32
-  { \msg_fatal:nnx { unravel } { not-implemented } { halign } }
+  { \@@_not_implemented:n { halign } }
 \@@_new_tex_cmd:nn { valign }                               % 33
-  { \msg_fatal:nnx { unravel } { not-implemented } { valign } }
+  { \@@_not_implemented:n { valign } }
 \@@_new_tex_cmd:nn { no_align }                             % 34
-  { \msg_fatal:nnx { unravel } { not-implemented } { noalign } }
+  { \@@_not_implemented:n { noalign } }
 %    \end{macrocode}
 %
 %    \begin{macrocode}
@@ -5187,10 +5291,10 @@
   { \@@_mode_vertical:n { \@@_do_rule: } }
 \cs_new_protected_nopar:Npn \@@_do_rule:
   {
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpush:N \l_@@_head_tl
     \@@_print_action:
     \@@_scan_alt_rule:
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpop:N \l_@@_head_tl
     \tl_use:N \l_@@_head_tl \scan_stop:
     \@@_print_action:x { \tl_to_str:N \l_@@_head_tl }
   }
@@ -5212,11 +5316,12 @@
   {
     \token_if_eq_meaning:NNTF \l_@@_head_token \tex_ignorespaces:D
       {
+        \@@_print_action:
         \@@_get_x_non_blank:
         \@@_set_cmd:
         \@@_do_step:
       }
-      { \msg_error:nn { unravel } { not-implemented } { pdfprimitive } }
+      { \@@_not_implemented:n { pdfprimitive } }
   }
 %    \end{macrocode}
 %
@@ -5267,10 +5372,10 @@
 %    \begin{macrocode}
 \@@_new_tex_cmd:nn { break_penalty }                        % 42
   {
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpush:N \l_@@_head_tl
     \@@_print_action:
     \@@_scan_int:
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpop:N \l_@@_head_tl
     \tl_use:N \l_@@_head_tl \scan_stop:
     \@@_print_action:x { \tl_to_str:N \l_@@_head_tl }
   }
@@ -5304,11 +5409,11 @@
   }
 %    \end{macrocode}
 %
-% \begin{macro}[aux]{\@@_do_accent:}
+% \begin{macro}{\@@_do_accent:}
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_do_accent:
   {
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpush:N \l_@@_head_tl
     \@@_print_action:
     \@@_scan_int:
     \@@_do_assignments:
@@ -5332,7 +5437,7 @@
           }
           { \@@_break:w }
       }
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpop:N \l_@@_head_tl
     \gtl_gput_right:NV \g_@@_output_gtl \l_@@_head_tl
     \tl_use:N \l_@@_head_tl \scan_stop:
     \@@_print_action:x { \tl_to_str:N \l_@@_head_tl }
@@ -5341,17 +5446,17 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_do_math_accent:}
+% \begin{macro}{\@@_do_math_accent:}
 %   \TeX{} will complain if \cs{l_@@_head_tl} happens to start with
 %   \tn{accent} (the user used \tn{accent} in math mode).
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_do_math_accent:
   {
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpush:N \l_@@_head_tl
     \@@_print_action:
     \@@_scan_int:
     \@@_scan_math:
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpop:N \l_@@_head_tl
     \gtl_gput_right:NV \g_@@_output_gtl \l_@@_head_tl
     \tl_use:N \l_@@_head_tl \scan_stop:
     \@@_print_action:x { \tl_to_str:N \l_@@_head_tl }
@@ -5377,7 +5482,7 @@
 %
 %    \begin{macrocode}
 \@@_new_tex_cmd:nn { discretionary }                        % 47
-  { \msg_error:nnx { unravel } { not-implemented } { discretionary } }
+  { \@@_not_implemented:n { discretionary } }
 %    \end{macrocode}
 %
 % \subsubsection{Maths: from 48 to 56}
@@ -5396,47 +5501,47 @@
 %
 %    \begin{macrocode}
 \@@_new_tex_cmd:nn { eq_no }                                % 48
-  { \msg_error:nnx { unravel } { not-implemented } { eqno } }
+  { \@@_not_implemented:n { eqno } }
 %    \end{macrocode}
 %
 %    \begin{macrocode}
 \@@_new_tex_cmd:nn { left_right }                           % 49
-  { \msg_error:nnx { unravel } { not-implemented } { left/right } }
+  { \@@_not_implemented:n { left/right } }
 %    \end{macrocode}
 %
 %    \begin{macrocode}
 \@@_new_tex_cmd:nn { math_comp }                            % 50
-  { \msg_error:nnx { unravel } { not-implemented } { math~comp } }
+  { \@@_not_implemented:n { math~comp } }
 %    \end{macrocode}
 %
 %    \begin{macrocode}
 \@@_new_tex_cmd:nn { limit_switch }                         % 51
-  { \msg_error:nnx { unravel } { not-implemented } { limits } }
+  { \@@_not_implemented:n { limits } }
 %    \end{macrocode}
 %
 %    \begin{macrocode}
 \@@_new_tex_cmd:nn { above }                                % 52
-  { \msg_error:nnx { unravel } { not-implemented } { above } }
+  { \@@_not_implemented:n { above } }
 %    \end{macrocode}
 %
 %    \begin{macrocode}
 \@@_new_tex_cmd:nn { math_style }                           % 53
-  { \msg_error:nnx { unravel } { not-implemented } { math~style } }
+  { \@@_not_implemented:n { math~style } }
 %    \end{macrocode}
 %
 %    \begin{macrocode}
 \@@_new_tex_cmd:nn { math_choice }                          % 54
-  { \msg_error:nnx { unravel } { not-implemented } { math~choice } }
+  { \@@_not_implemented:n { math~choice } }
 %    \end{macrocode}
 %
 %    \begin{macrocode}
 \@@_new_tex_cmd:nn { non_script }                           % 55
-  { \msg_error:nnx { unravel } { not-implemented } { non~script } }
+  { \@@_not_implemented:n { non~script } }
 %    \end{macrocode}
 %
 %    \begin{macrocode}
 \@@_new_tex_cmd:nn { vcenter }                              % 56
-  { \msg_error:nnx { unravel } { not-implemented } { vcenter } }
+  { \@@_not_implemented:n { vcenter } }
 %    \end{macrocode}
 %
 % \subsubsection{From 57 to 70}
@@ -5461,9 +5566,9 @@
 %    \begin{macrocode}
 \@@_new_tex_cmd:nn { case_shift }                           % 57
   {
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpush:N \l_@@_head_tl
     \@@_scan_toks:NN \c_false_bool \c_false_bool
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_tmpa_tl
+    \@@_prev_input_gpop:N \l_@@_tmpa_tl
     \exp_after:wN \@@_case_shift:Nn \l_@@_tmpa_tl
   }
 \cs_new_protected:Npn \@@_case_shift:Nn #1#2
@@ -5477,10 +5582,10 @@
 %    \begin{macrocode}
 \@@_new_tex_cmd:nn { message }                              % 58
   {
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpush:N \l_@@_head_tl
     \@@_print_action:
     \@@_scan_toks_to_str:
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpop:N \l_@@_head_tl
     \tl_use:N \l_@@_head_tl
     \@@_print_action:x { \tl_to_str:N \l_@@_head_tl }
   }
@@ -5490,10 +5595,10 @@
 %    \begin{macrocode}
 \@@_new_tex_cmd:nn { extension }                            % 59
   {
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpush:N \l_@@_head_tl
     \@@_print_action:
     \@@_scan_extension_operands:
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpop:N \l_@@_head_tl
     \tl_use:N \l_@@_head_tl \scan_stop:
     \@@_print_action:x { \tl_to_str:N \l_@@_head_tl }
   }
@@ -5502,7 +5607,7 @@
 %    \begin{macrocode}
 \@@_new_tex_cmd:nn { in_stream }                            % 60
   {
-    \seq_put_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpush:N \l_@@_head_tl
     \@@_print_action:
     \token_if_eq_meaning:NNTF \l_@@_head_token \tex_openin:D
       {
@@ -5511,7 +5616,7 @@
         \@@_scan_file_name:
       }
       { \@@_scan_int: }
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpop:N \l_@@_head_tl
     \tl_use:N \l_@@_head_tl \scan_stop:
     \@@_print_action:x { \tl_to_str:N \l_@@_head_tl }
   }
@@ -5536,7 +5641,7 @@
 %
 %    \begin{macrocode}
 \@@_new_tex_cmd:nn { omit }                                 % 63
-  { \msg_error:nn { unravel } { not-implemented } { omit } }
+  { \@@_not_implemented:n { omit } }
 %    \end{macrocode}
 %
 %    \begin{macrocode}
@@ -5560,11 +5665,11 @@
   {
     \@@_mode_math:n
       {
-        \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpush:N \l_@@_head_tl
         \@@_print_action:
         \@@_scan_int:
         \@@_scan_math:
-        \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpop:N \l_@@_head_tl
         \gtl_gput_right:NV \g_@@_output_gtl \l_@@_head_tl
         \tl_use:N \l_@@_head_tl \scan_stop:
         \@@_print_action:x { \tl_to_str:N \l_@@_head_tl }
@@ -5572,10 +5677,12 @@
   }
 %    \end{macrocode}
 %
-% Let \TeX{} cause the error.
 %    \begin{macrocode}
 \@@_new_tex_cmd:nn { end_cs_name }                          % 67
-  { \l_@@_head_token \@@_print_action: }
+  {
+    \@@_tex_error:nV { extra-endcsname } \l_@@_head_tl
+    \@@_print_action:
+  }
 %    \end{macrocode}
 %
 % See |the_char| and |other_char|.
@@ -5635,7 +5742,7 @@
           {
             \mode_if_horizontal:TF
               { \@@_scan_int: }
-              { \msg_error:nn { unravel } { invalid-mode } }
+              { \@@_error:nnnnn { invalid-mode } { } { } { } { } }
           }
         { 6 } % pdfliteral
           {
@@ -5691,7 +5798,7 @@
         { 14 } % pdfstartlink
           {
             \mode_if_vertical:TF
-              { \msg_error:nn { unravel } { invalid-mode } }
+              { \@@_error:nnnnn { invalid-mode } { } { } { } { } }
               {
                 \@@_scan_rule_attr:
                 \@@_scan_action:
@@ -5700,7 +5807,7 @@
         { 15 } % pdfendlink
           {
             \mode_if_vertical:T
-              { \msg_error:nn { unravel } { invalid-mode } }
+              { \@@_error:nnnnn { invalid-mode } { } { } { } { } }
           }
         { 16 } % pdfoutline
           {
@@ -5788,7 +5895,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_scan_pdfcolorstack_operands:}
+% \begin{macro}{\@@_scan_pdfcolorstack_operands:}
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_scan_pdfcolorstack_operands:
   {
@@ -5801,8 +5908,8 @@
               {
                 \@@_scan_keyword:nF { cCuUrRrReEnNtT }
                   {
-                    \msg_error:nn { unravel }
-                      { color-stack-action-missing }
+                    \@@_error:nnnnn { color-stack-action-missing }
+                      { } { } { } { }
                   }
               }
           }
@@ -5811,7 +5918,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_scan_rule_attr:}
+% \begin{macro}{\@@_scan_rule_attr:}
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_scan_rule_attr:
   {
@@ -5832,7 +5939,7 @@
         \@@_scan_keyword:nF { gGoOtToO }
           {
             \@@_scan_keyword:nF { tThHrReEaAdD }
-              { \msg_error:nn { unravel } { action-type-missing } }
+              { \@@_error:nnnnn { action-type-missing } { } { } { } { } }
           }
       }
     \@@_scan_keyword:nT { fFiIlLeE }
@@ -5848,7 +5955,7 @@
           {
             \@@_scan_keyword:nTF { nNuUmM }
               { \@@_scan_int: }
-              { \msg_error:nn { unravel } { identifier-type-missing } }
+              { \@@_error:nnnnn { identifier-type-missing } { } { } { } { } }
           }
       }
     \@@_scan_keyword:nTF { nNeEwWwWiInNdDoOwW }
@@ -5861,7 +5968,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_scan_image:}
+% \begin{macro}{\@@_scan_image:}
 %   Used by \tn{pdfximage}.
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_scan_image:
@@ -5912,15 +6019,15 @@
 \cs_new_protected_nopar:Npn \@@_scan_immediate_operands_bad:
   {
     \@@_back_input:
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpop:N \l_@@_head_tl
     \@@_print_action:x { \tl_to_str:N \l_@@_head_tl ignored }
-    \seq_gput_right:Nn \g_@@_prev_input_seq { }
+    \@@_prev_input_gpush:
   }
 
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_scan_pdfdest_operands:}
+% \begin{macro}{\@@_scan_pdfdest_operands:}
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_scan_pdfdest_operands:
   {
@@ -5929,7 +6036,7 @@
       {
         \@@_scan_keyword:nTF { nNaAmMeE }
           { \@@_scan_pdf_ext_toks: }
-          { \msg_error:nn { unravel } { identifier-type-missing } }
+          { \@@_error:nnnnn { identifier-type-missing } { } { } { } { } }
       }
     \@@_scan_keyword:nTF { xXyYzZ }
       {
@@ -5958,10 +6065,8 @@
                                 \@@_scan_keyword:nF
                                   { fFiItT }
                                   {
-                                    \msg_error:nn { unravel }
-                                      {
-                                        destination-type-missing
-                                      }
+                                    \@@_error:nnnnn { destination-type-missing }
+                                      { } { } { } { }
                                   }
                               }
                           }
@@ -5986,7 +6091,7 @@
 %    \begin{macrocode}
 \cs_set_protected_nopar:Npn \@@_tmp:w
   {
-    \seq_gput_right:Nn \g_@@_prev_input_seq { }
+    \@@_prev_input_gpush:
     \@@_prefixed_command:
   }
 \int_step_inline:nnnn
@@ -5998,7 +6103,7 @@
 %
 % \begin{macro}[int]{\@@_prefixed_command:}
 %   Accumulated prefix codes so far are stored as the last item of
-%   \cs{g_@@_prev_input_seq}.
+%   the previous-input sequence.
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_prefixed_command:
   {
@@ -6011,10 +6116,11 @@
         \int_compare:nNnF \l_@@_head_cmd_int
           > { \@@_tex_use:n { max_non_prefixed_command } }
           {
-            \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_tmpa_tl
-            \msg_error:nnxx { unravel  } { erroneous-prefixes }
+            \@@_prev_input_gpop:N \l_@@_tmpa_tl
+            \@@_error:nxxxx { erroneous-prefixes }
               { \tl_to_str:N \l_@@_tmpa_tl }
               { \tl_to_str:N l_@@_head_tl }
+              { } { }
             \@@_back_input:
             \@@_omit_after_assignment:w
           }
@@ -6024,7 +6130,7 @@
     \cs_if_exist_use:cF
       { @@_prefixed_ \int_use:N \l_@@_head_cmd_int : }
       {
-        \msg_error:nnx { unravel } { internal } { prefixed }
+        \@@_error:nnnnn { internal } { prefixed } { } { } { }
         \@@_omit_after_assignment:w
       }
     \@@_after_assignment:
@@ -6035,8 +6141,8 @@
 % range $[71,102]$, with the exception of |prefix=93|, which would have
 % been collected by the \cs{@@_prefixed_command:} loop.
 %
-% \begin{macro}[aux]{\@@_after_assignment:}
-% \begin{macro}[aux]{\@@_omit_after_assignment:w}
+% \begin{macro}{\@@_after_assignment:}
+% \begin{macro}{\@@_omit_after_assignment:w}
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_after_assignment:
   {
@@ -6063,7 +6169,7 @@
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_assign_token:n #1
   {
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpop:N \l_@@_head_tl
     #1
     \tl_use:N \l_@@_head_tl \scan_stop:
     \@@_print_assigned_token:
@@ -6075,7 +6181,7 @@
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_assign_register:
   {
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpop:N \l_@@_head_tl
     \tl_use:N \l_@@_head_tl \scan_stop:
     \@@_print_assigned_register:
   }
@@ -6088,10 +6194,10 @@
   {
     \tl_if_empty:nF {#1}
       {
-        \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpush:N \l_@@_head_tl
         \@@_print_action:x { \tl_to_str:N \l_@@_head_tl }
         #1
-        \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpop:N \l_@@_head_tl
       }
     \@@_prev_input:V \l_@@_head_tl
     \tl_set_eq:NN \l_@@_defined_tl \l_@@_head_tl
@@ -6102,16 +6208,16 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_assign_toks:}
+% \begin{macro}{\@@_assign_toks:}
 %    \begin{macrocode}
 \@@_prefixed_new:nn { toks_register }                       % 71
   {
     \int_compare:nNnT \l_@@_head_char_int = \c_zero
       { % \toks
-        \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpush:N \l_@@_head_tl
         \@@_print_action:
         \@@_scan_int:
-        \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpop:N \l_@@_head_tl
       }
     \@@_assign_toks:
   }
@@ -6214,7 +6320,7 @@
   }
 \@@_prefixed_new:nn { set_font }                            % 87
   {
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_tmpa_tl
+    \@@_prev_input_gpop:N \l_@@_tmpa_tl
     \tl_put_left:NV \l_@@_head_tl \l_@@_tmpa_tl
     \tl_use:N \l_@@_head_tl \scan_stop:
     \gtl_gput_right:NV \g_@@_output_gtl \l_@@_head_tl
@@ -6249,11 +6355,11 @@
 %    \begin{macrocode}
 \@@_prefixed_new:nn { let }                                 % 94
   {
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpush:N \l_@@_head_tl
     \token_if_eq_meaning:NNTF \l_@@_head_token \tex_let:D
       { % |let|
         \@@_scan_r_token:
-        \seq_get_right:NN \g_@@_prev_input_seq \l_@@_tmpa_tl
+        \@@_prev_input_get:N \l_@@_tmpa_tl
         \@@_print_action:x { \tl_to_str:N \l_@@_tmpa_tl }
         \@@_get_next:
         \bool_while_do:nn
@@ -6266,7 +6372,7 @@
       }
       { % |futurelet|
         \@@_scan_r_token:
-        \seq_get_right:NN \g_@@_prev_input_seq \l_@@_tmpa_tl
+        \@@_prev_input_get:N \l_@@_tmpa_tl
         \@@_print_action:x { \tl_to_str:N \l_@@_tmpa_tl }
         \@@_get_next:
         \gtl_set_eq:NN \l_@@_tmpb_gtl \l_@@_head_gtl
@@ -6275,10 +6381,10 @@
         \gtl_set_eq:NN \l_@@_head_gtl \l_@@_tmpb_gtl
         \@@_back_input:
       }
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_tmpa_tl
+    \@@_prev_input_gpop:N \l_@@_tmpa_tl
     \tl_put_right:Nn \l_@@_tmpa_tl { = ~ \l_@@_head_token }
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
-    \use:x
+    \@@_prev_input_gpop:N \l_@@_head_tl
+    \@@_exp_args:Nx \use:n
       {
         \exp_not:V \l_@@_head_tl
         \tex_let:D \tl_tail:N \l_@@_tmpa_tl
@@ -6304,7 +6410,7 @@
   }
 %    \end{macrocode}
 %
-% \begin{macro}[aux, EXP]{\@@_read_to_cs_safe:nTF, \@@_read_to_cs_safe:fTF}
+% \begin{macro}[EXP]{\@@_read_to_cs_safe:nTF, \@@_read_to_cs_safe:fTF}
 %   After \tn{read} or \tn{readline}, find an int, the mandatory
 %   keyword |to|, and an assignable token.  The \tn{read} and
 %   \tn{readline} primitives throw a fatal error in \tn{nonstopmode}
@@ -6321,12 +6427,12 @@
     \@@_scan_int:
     \@@_scan_to:
     \@@_scan_r_token:
-    \seq_get_right:NN \g_@@_prev_input_seq \l_@@_tmpa_tl
+    \@@_prev_input_get:N \l_@@_tmpa_tl
     \@@_read_to_cs_safe:fTF
       { \@@_tl_first_int:N \l_@@_tmpa_tl }
       { \@@_assign_token:n { } }
       {
-        \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpop:N \l_@@_head_tl
         \@@_tex_fatal_error:nV { cannot-read } \l_@@_head_tl
       }
   }
@@ -6357,10 +6463,10 @@
 %    \begin{macrocode}
 \@@_prefixed_new:nn { def }                                 % 97
   {
-    \seq_get_right:NN \g_@@_prev_input_seq \l_@@_tmpa_tl
+    \@@_prev_input_get:N \l_@@_tmpa_tl
     \tl_set:NV \l_@@_defining_tl \l_@@_tmpa_tl
     \tl_put_right:NV \l_@@_defining_tl \l_@@_head_tl
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpush:N \l_@@_head_tl
     \int_compare:nNnTF \l_@@_head_char_int < \c_two
       { % def/gdef
         \@@_scan_r_token:
@@ -6372,7 +6478,7 @@
         \tl_put_right:NV \l_@@_defining_tl \l_@@_defined_tl
         \@@_scan_toks:NN \c_true_bool \c_true_bool
       }
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpop:N \l_@@_head_tl
     \@@_prev_input:V \l_@@_head_tl
     \@@_assign_token:n
       { \tl_set_eq:NN \l_@@_head_tl \l_@@_defining_tl }
@@ -6380,7 +6486,7 @@
 %    \end{macrocode}
 %
 % \tn{setbox} is a bit special: directly put it in
-% \cs{g_@@_prev_input_seq} with the prefixes; the box code will take
+% the previous-input sequence with the prefixes; the box code will take
 % care of things, and expects a single item containing what it needs to
 % do.
 %    \begin{macrocode}
@@ -6392,8 +6498,8 @@
     \bool_if:NTF \g_@@_set_box_allowed_bool
       { \@@_do_box:N \c_false_bool }
       {
-        \msg_error:nn { unravel } { improper-setbox }
-        \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_tmpa_tl
+        \@@_error:nnnnn { improper-setbox } { } { } { } { }
+        \@@_prev_input_gpop:N \l_@@_tmpa_tl
         \@@_omit_after_assignment:w
       }
   }
@@ -6413,7 +6519,7 @@
 %    \begin{macrocode}
 \@@_prefixed_new:nn { set_interaction }                     % 100
   {
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_tmpa_tl
+    \@@_prev_input_gpop:N \l_@@_tmpa_tl
     \tl_put_left:NV \l_@@_head_tl \l_@@_tmpa_tl
     \tl_use:N \l_@@_head_tl \scan_stop:
     \@@_print_action:x { \tl_to_str:N \l_@@_head_tl }
@@ -6466,7 +6572,7 @@
   { \@@_do_operation:N \c_three }
 %    \end{macrocode}
 %
-% \begin{macro}[aux]{\@@_do_operation:N, \@@_do_operation_fail:w}
+% \begin{macro}{\@@_do_operation:N, \@@_do_operation_fail:w}
 %   \begin{macrocode}
 \cs_new_protected:Npn \@@_do_operation:N #1
   {
@@ -6501,14 +6607,14 @@
   }
 \cs_new_protected_nopar:Npn \@@_do_operation_fail:w
   {
-    \msg_error:nn { unravel } { after-advance }
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_tmpa_tl
+    \@@_error:nnnnn { after-advance } { } { } { } { }
+    \@@_prev_input_gpop:N \l_@@_tmpa_tl
     \@@_omit_after_assignment:w
   }
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_do_register:N, \@@_do_register_aux:Nn}
+% \begin{macro}{\@@_do_register:N, \@@_do_register_aux:Nn}
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_do_register:N #1
   {
@@ -6519,10 +6625,10 @@
   {
     \int_compare:nNnTF { \tl_tail:n {#2} } = \c_zero
       {
-        \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpush:N \l_@@_head_tl
         \@@_print_action:
         \@@_scan_int:
-        \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpop:N \l_@@_head_tl
         \@@_prev_input_silent:V \l_@@_head_tl
       }
       {
@@ -6536,7 +6642,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_do_register_set:Nn}
+% \begin{macro}{\@@_do_register_set:Nn}
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_do_register_set:Nn #1#2
   {
@@ -6557,7 +6663,7 @@
             { 3 } { \@@_scan_normal_glue:  } % glue
             { 4 } { \@@_scan_mu_glue:      } % muglue
           }
-          { \msg_error:nnx { unravel } { internal } { do-reg=#2 } }
+          { \@@_error:nxxxx { internal } { do-reg=#2 } { } { } { } }
       }
       { \@@_scan_int: }
     \@@_assign_register:
@@ -6576,7 +6682,7 @@
       > { \@@_tex_use:n { max_non_prefixed_command } }
       {
         \bool_gset_false:N \g_@@_set_box_allowed_bool
-        \seq_gput_right:Nn \g_@@_prev_input_seq { }
+        \@@_prev_input_gpush:
         \@@_prefixed_command:
         \bool_gset_true:N \g_@@_set_box_allowed_bool
         \@@_do_assignments:
@@ -6628,7 +6734,7 @@
   { \tl_use:N \l_@@_head_tl \@@_print_action: }
 %    \end{macrocode}
 %
-% \begin{macro}[aux]{\@@_expandafter:, \@@_unless:, \@@_unless_bad:}
+% \begin{macro}{\@@_expandafter:, \@@_unless:, \@@_unless_bad:}
 %    \begin{macrocode}
 \@@_new_tex_expandable:nn { expand_after }                  % 104
   {
@@ -6641,13 +6747,13 @@
     \@@_get_next:
     \gtl_concat:NNN \l_@@_head_gtl
       \l_@@_tmpb_gtl \l_@@_head_gtl
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_gtl
+    \@@_prev_input_gpush_gtl:N \l_@@_head_gtl
     \@@_print_action:x { \gtl_to_str:N \l_@@_head_gtl }
     \@@_get_next:
     \@@_token_if_expandable:NTF \l_@@_head_token
       { \@@_expand: }
       { \@@_back_input: }
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_gtl
+    \@@_prev_input_gpop:N \l_@@_head_gtl
     \@@_set_action_text:x
       { back_input: ~ \gtl_to_str:N \l_@@_head_gtl }
     \gtl_pop_left:N \l_@@_head_gtl
@@ -6672,13 +6778,13 @@
   }
 \cs_new_protected_nopar:Npn \@@_unless_bad:
   {
-    \msg_error:nn { unravel } { bad-unless }
+    \@@_error:nnnnn { bad-unless } { } { } { } { }
     \@@_back_input:
   }
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_noexpand:, \@@_pdfprimitive:}
+% \begin{macro}{\@@_noexpand:, \@@_pdfprimitive:}
 %    \begin{macrocode}
 \@@_new_tex_expandable:nn { no_expand }                     % 105
   {
@@ -6702,11 +6808,11 @@
       }
   }
 \cs_new_protected_nopar:Npn \@@_pdfprimitive:
-  { \msg_error:nnx { unravel } { not-implemented } { pdfprimitive } }
+  { \@@_not_implemented:n { pdfprimitive } }
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_endinput:, \@@_scantokens:, \@@_input:}
+% \begin{macro}{\@@_endinput:, \@@_scantokens:, \@@_input:}
 %    \begin{macrocode}
 \@@_new_tex_expandable:nn { input }                       % 106
   {
@@ -6722,23 +6828,25 @@
   }
 \cs_new_protected_nopar:Npn \@@_endinput:
   {
-    \msg_warning:nn { unravel } { endinput-ignored }
+    \group_begin:
+      \msg_warning:nn { unravel } { endinput-ignored }
+    \group_end:
     \@@_print_action:
   }
 \cs_new_protected_nopar:Npn \@@_scantokens:
   {
-    \seq_gput_right:Nn \g_@@_prev_input_seq { }
+    \@@_prev_input_gpush:
     \@@_scan_toks:NN \c_false_bool \c_false_bool
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_tmpa_tl
+    \@@_prev_input_gpop:N \l_@@_tmpa_tl
     \tl_set_rescan:Nno \l_@@_head_tl { } \l_@@_tmpa_tl
     \@@_back_input:V \l_@@_head_tl
     \@@_print_action:x { \tl_to_str:N \l_@@_tmpa_tl }
   }
 \cs_new_protected_nopar:Npn \@@_input:
   {
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpush:N \l_@@_head_tl
     \@@_scan_file_name:
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpop:N \l_@@_head_tl
     \tl_set:Nx \l_@@_tmpa_tl { \tl_tail:N \l_@@_head_tl }
     \@@_file_get:nN \l_@@_tmpa_tl \l_@@_tmpa_tl
     \@@_back_input:V \l_@@_tmpa_tl
@@ -6747,26 +6855,26 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_csname_loop:}
+% \begin{macro}{\@@_csname_loop:}
 %    \begin{macrocode}
 \@@_new_tex_expandable:nn { cs_name }                       % 109
   {
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpush:N \l_@@_head_tl
     \@@_print_action:
     \@@_csname_loop:
-    \@@_prev_input:V \l_@@_head_tl
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_silent:V \l_@@_head_tl
+    \@@_prev_input_gpop:N \l_@@_head_tl
     \@@_back_input_tl_o:
   }
 \cs_new_protected_nopar:Npn \@@_csname_loop:
   {
     \@@_get_x_next:
-    \token_if_cs:NTF \l_@@_head_token
+    \@@_gtl_if_head_is_definable:NTF \l_@@_head_gtl
       {
         \cs_if_eq:NNF \l_@@_head_token \tex_endcsname:D
           {
-            \msg_error:nn { unravel } { missing-endcsname }
             \@@_back_input:
+            \@@_tex_error:nV { missing-endcsname } \l_@@_head_tl
             \tl_set:Nn \l_@@_head_tl { \tex_endcsname:D }
           }
       }
@@ -6782,7 +6890,7 @@
 %    \begin{macrocode}
 \@@_new_tex_expandable:nn { convert }                       % 110
   {
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpush:N \l_@@_head_tl
     \@@_print_action:
     \int_case:nn \l_@@_head_char_int
       {
@@ -6818,7 +6926,7 @@
         { 30 } \@@_scan_int:
         { 31 } \@@_scan_pdfximagebbox:
       }
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpop:N \l_@@_head_tl
     \@@_back_input_tl_o:
   }
 \cs_new_protected_nopar:Npn \@@_scan_pdfstrcmp:
@@ -6853,7 +6961,7 @@
   }
 %    \end{macrocode}
 %
-% \begin{macro}[aux]{\@@_get_the:}
+% \begin{macro}{\@@_get_the:}
 %    \begin{macrocode}
 \@@_new_tex_expandable:nn { the }                           % 111
   {
@@ -6864,18 +6972,18 @@
   }
 \cs_new_protected_nopar:Npn \@@_get_the:
   {
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpush:N \l_@@_head_tl
     \@@_print_action:
     \int_if_odd:nTF \l_@@_head_char_int
       { % \unexpanded, \detokenize
         \@@_scan_toks:NN \c_false_bool \c_false_bool
-        \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpop:N \l_@@_head_tl
         \@@_set_action_text:x { \tl_to_str:N \l_@@_head_tl }
       }
       { % \the
         \@@_get_x_next:
         \@@_scan_something_internal:n { 5 }
-        \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpop:N \l_@@_head_tl
         \@@_set_action_text:x
           {
             \tl_head:N \l_@@_head_tl
@@ -6896,7 +7004,7 @@
 %    \begin{macrocode}
 \@@_new_tex_expandable:nn { end_template }                  % 117
   {
-    \msg_error:nn { unravel } { not-implemented } { end-template }
+    \@@_not_implemented:n { end-template } { } { } { }
     \@@_back_input_tl_o:
   }
 %    \end{macrocode}
@@ -6906,7 +7014,7 @@
 %
 % ^^A todo: simply use \cs{@@_input_gpop:N} right away.
 % \begin{macro}[int]{\@@_pass_text:}
-% \begin{macro}[aux]{\@@_pass_text_done:w}
+% \begin{macro}{\@@_pass_text_done:w}
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_pass_text:
   {
@@ -6938,7 +7046,7 @@
 % \end{macro}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_pass_text_nested:}
+% \begin{macro}{\@@_pass_text_nested:}
 %   Again, if there is no more input we are in trouble.  The
 %   construction otherwise essentially results in
 %   \begin{quote}
@@ -6980,11 +7088,11 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_pass_text_empty:}
+% \begin{macro}{\@@_pass_text_empty:}
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_pass_text_empty:
   {
-    \msg_error:nn { unravel } { runaway-if }
+    \@@_error:nnnnn { runaway-if } { } { } { } { }
     \@@_exit:w
   }
 %    \end{macrocode}
@@ -7010,7 +7118,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_change_if_limit:nn}
+% \begin{macro}{\@@_change_if_limit:nn}
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_change_if_limit:nn #1#2
   {
@@ -7040,7 +7148,7 @@
   }
 %    \end{macrocode}
 %
-% \begin{macro}[aux]{\@@_cond_aux:nn}
+% \begin{macro}{\@@_cond_aux:nn}
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_cond_aux:n #1
   {
@@ -7051,7 +7159,7 @@
         { 21 } { \@@_test_pdfprimitive:n {#1} } % ^^A todo and \unless
       }
       {
-        \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpush:N \l_@@_head_tl
         \@@_print_action:
         \int_case:nn \l_@@_head_char_int
           {
@@ -7082,7 +7190,7 @@
             { 23 } % ifpdfabsdim
               { \@@_test_two_vals:N \@@_scan_normal_dimen: }
           }
-        \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+        \@@_prev_input_gpop:N \l_@@_head_tl
         \@@_set_action_text:x { \tl_to_str:N \l_@@_head_tl }
         \l_@@_head_tl \scan_stop:
           \exp_after:wN \@@_cond_true:n
@@ -7095,7 +7203,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_cond_true:n}
+% \begin{macro}{\@@_cond_true:n}
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_cond_true:n #1
   {
@@ -7105,7 +7213,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]
+% \begin{macro}
 %   {\@@_cond_false:n, \@@_cond_false_loop:n, \@@_cond_false_common:}
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_cond_false:n #1
@@ -7121,7 +7229,7 @@
       {
         \token_if_eq_meaning:NNT \l_@@_head_token \or:
           {
-            \msg_error:nn { unravel } { extra-or }
+            \@@_error:nnnnn { extra-or } { } { } { } { }
             \@@_cond_false_loop:n {#1}
           }
       }
@@ -7140,7 +7248,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_test_two_vals:N}
+% \begin{macro}{\@@_test_two_vals:N}
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_test_two_vals:N #1
   {
@@ -7148,7 +7256,7 @@
     \@@_get_x_non_blank:
     \tl_if_in:nVF { < = > } \l_@@_head_tl
       {
-        \msg_error:nn { unravel } { missing-equals }
+        \@@_error:nnnnn { missing-equals } { } { } { } { }
         \@@_back_input:
         \tl_set:Nn \l_@@_head_tl { = }
       }
@@ -7160,7 +7268,7 @@
 %
 % ^^A todo: fix interaction with noexpand and active chars
 % ^^A todo: fix non-brace begin-group and end-group tokens
-% \begin{macro}[aux]{\@@_test_two_chars:, \@@_test_two_chars_aux:}
+% \begin{macro}{\@@_test_two_chars:, \@@_test_two_chars_aux:}
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_test_two_chars:
   {
@@ -7185,16 +7293,16 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_test_ifx:n, \@@_test_ifx_aux:w}
+% \begin{macro}{\@@_test_ifx:n, \@@_test_ifx_aux:w}
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_test_ifx:n #1
   {
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpush:N \l_@@_head_tl
     \@@_print_action:
     \@@_get_next:
     \gtl_set_eq:NN \l_@@_tmpb_gtl \l_@@_head_gtl
     \@@_get_next:
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_tmpa_tl
+    \@@_prev_input_gpop:N \l_@@_tmpa_tl
     \@@_set_action_text:x
       {
         Compare:~ \tl_to_str:N \l_@@_tmpa_tl
@@ -7213,19 +7321,19 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_test_case:n, \@@_test_case_aux:nn}
+% \begin{macro}{\@@_test_case:n, \@@_test_case_aux:nn}
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_test_case:n #1
   {
-    \seq_gput_right:NV \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpush:N \l_@@_head_tl
     \@@_print_action:
     \bool_if:NT \g_@@_internal_debug_bool { \iow_term:n { {\ifcase level~#1} } }
     \@@_scan_int:
-    \seq_get_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_get:N \l_@@_head_tl
     \tl_set:Nx \l_@@_head_tl { \tl_tail:N \l_@@_head_tl }
     % ^^A does text_case_aux use prev_input_seq?
     \exp_args:No \@@_test_case_aux:nn { \l_@@_head_tl } {#1}
-    \seq_gpop_right:NN \g_@@_prev_input_seq \l_@@_head_tl
+    \@@_prev_input_gpop:N \l_@@_head_tl
     \@@_print_action:x { \tl_to_str:N \l_@@_head_tl }
   }
 \cs_new_protected:Npn \@@_test_case_aux:nn #1#2
@@ -7253,7 +7361,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_test_ifdefined:}
+% \begin{macro}{\@@_test_ifdefined:}
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_test_ifdefined:
   {
@@ -7277,7 +7385,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_test_ifcsname:}
+% \begin{macro}{\@@_test_ifcsname:}
 %    \begin{macrocode}
 \cs_new_protected_nopar:Npn \@@_test_ifcsname:
   {
@@ -7295,10 +7403,10 @@
         \int_compare:nNnTF \g_@@_if_limit_int = \c_zero
           {
             \int_compare:nNnTF \g_@@_if_depth_int = \c_zero
-              { \msg_error:nn { unravel } { extra-fi-or-else } }
+              { \@@_error:nnnnn { extra-fi-or-else } { } { } { } { } }
               { \@@_insert_relax: }
           }
-          { \msg_error:nn { unravel } { extra-fi-or-else } }
+          { \@@_error:nnnnn { extra-fi-or-else } { } { } { } { } }
       }
       {
         \@@_set_action_text:
@@ -7336,10 +7444,36 @@
 % Let us start with the procedure which prints to the terminal: this
 % will help me test the code while I'm writing it.
 %
+% \begin{macro}{\@@_print_normalize_null:}
+% \begin{variable}{\l_@@_print_tl}
+%   Change the null character to an explicit |^||^||@| in \LuaTeX{} to
+%   avoid a bug whereby a null character ends a string prematurely.
+%    \begin{macrocode}
+\tl_new:N \l_@@_print_tl
+\sys_if_engine_luatex:TF
+  {
+    \cs_new_protected:Npx \@@_print_normalize_null:
+      {
+        \tl_replace_all:Nnn \exp_not:N \l_@@_print_tl
+          { \char_generate:nn { 0 } { 12 } }
+          { \tl_to_str:n { ^ ^ @ } }
+      }
+  }
+  { \cs_new_protected:Npn \@@_print_normalize_null: { } }
+%    \end{macrocode}
+% \end{variable}
+% \end{macro}
+%
 % \begin{macro}[int]{\@@_print:n, \@@_print:x}
 %    \begin{macrocode}
-\cs_new_eq:NN \@@_print:n \iow_term:n
-\cs_generate_variant:Nn \@@_print:n { x }
+\cs_new_protected:Npn \@@_print:n #1
+  {
+    \tl_set:Nn \l_@@_print_tl {#1}
+    \@@_print_normalize_null:
+    \@@_exp_args:Nx \iow_term:n { \l_@@_print_tl }
+  }
+\cs_new_protected:Npn \@@_print:x
+  { \@@_exp_args:Nx \@@_print:n }
 %    \end{macrocode}
 % \end{macro}
 %
@@ -7388,15 +7522,9 @@
       \tl_use:N \g_@@_before_print_state_tl
       \int_compare:nNnT \g_@@_noise_int > \c_zero
         {
-          \exp_args:Nx \@@_print_state_output:n
-            { \gtl_to_str:N \g_@@_output_gtl }
-          \seq_set_map:NNn \l_@@_tmpa_seq \g_@@_prev_input_seq
-            { \@@_to_str:n {##1} }
-          \seq_remove_all:Nn \l_@@_tmpa_seq { }
-          \exp_args:Nx \@@_print_state_prev:n
-            { \seq_use:Nn \l_@@_tmpa_seq { \\ } }
-          \exp_args:Nx \@@_print_state_input:n
-            { \@@_input_to_str: }
+          \@@_print_state_output:
+          \@@_print_state_prev:
+          \@@_print_state_input:
         }
     \group_end:
     \@@_prompt:
@@ -7404,11 +7532,16 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_print_state_output:n}
+% \begin{macro}{\@@_print_state_output:, \@@_print_state_output:n}
 %   Unless empty, print |#1| with each line starting with \verb*+<|~+.
 %   The \cs{@@_str_truncate_left:nn} function trims |#1| if needed, to
 %   fit in a maximum of \cs{g_@@_max_output_int} characters.
 %    \begin{macrocode}
+\cs_new_protected_nopar:Npn \@@_print_state_output:
+  {
+    \@@_exp_args:Nx \@@_print_state_output:n
+      { \gtl_to_str:N \g_@@_output_gtl }
+  }
 \cs_new_protected:Npn \@@_print_state_output:n #1
   {
     \tl_if_empty:nF {#1}
@@ -7420,19 +7553,36 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_print_state_prev:n}
-%   Never trim~|#1|.
+% \begin{macro}{\@@_print_state_prev:}
+%   Never trim~|##1|.
 %    \begin{macrocode}
-\cs_new_protected:Npn \@@_print_state_prev:n #1
-  { \@@_print_message:nn { || ~ } {#1} }
+\cs_new_protected_nopar:Npn \@@_print_state_prev:
+  {
+    \seq_set_map:NNn \l_@@_tmpa_seq \g_@@_prev_input_seq
+      { \@@_to_str:n {##1} }
+    \seq_remove_all:Nn \l_@@_tmpa_seq { }
+    \seq_if_empty:NTF \l_@@_tmpa_seq
+      { \@@_print_message:nn { || ~ } { } }
+      {
+        \seq_map_inline:Nn \l_@@_tmpa_seq
+          {
+            \@@_print_message:nn { || ~ } {##1}
+          }
+      }
+  }
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_print_state_input:n}
+% \begin{macro}{\@@_print_state_input:, \@@_print_state_input:n}
 %   Print |#1| with each line starting with \verb*+|>~+.  The
 %   \cs{@@_str_truncate_right:nn} function trims |#1| if needed, to fit
 %   in a maximum of \cs{g_@@_max_input_int} characters.
 %    \begin{macrocode}
+\cs_new_protected_nopar:Npn \@@_print_state_input:
+  {
+    \@@_exp_args:Nx \@@_print_state_input:n
+      { \@@_input_to_str: }
+  }
 \cs_new_protected:Npn \@@_print_state_input:n #1
   {
     \@@_print_message:nn { |> ~ }
@@ -7441,7 +7591,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_print_meaning:}
+% \begin{macro}{\@@_print_meaning:}
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_print_meaning:
   {
@@ -7519,7 +7669,7 @@
 \cs_new_protected_nopar:Npn \@@_print_assigned_register:
   {
     \@@_after_assignment: % ^^A todo: simplify
-    \exp_args:Nx \@@_print_action:x
+    \@@_exp_args:Nx \@@_print_action:x
       {
         \exp_not:n
           {
@@ -7590,8 +7740,8 @@
     \int_compare:nNnT { \etex_interactionmode:D } = { 3 }
       {
         \bool_if:NTF \g_@@_explicit_prompt_bool
-          { \ior_get_str:Nc \c_@@_prompt_ior }
-          { \ior_get_str:Nc \c_@@_noprompt_ior }
+          { \ior_str_get:Nc \c_@@_prompt_ior }
+          { \ior_str_get:Nc \c_@@_noprompt_ior }
               { Your~input }
         \exp_args:Nv \@@_prompt_treat:n { Your~input }
       }
@@ -7600,7 +7750,7 @@
   {
     \tl_if_empty:nF {#1}
       {
-        \exp_args:Nx \str_case:nnF { \tl_head:n {#1} }
+        \@@_exp_args:Nx \str_case:nnF { \tl_head:n {#1} }
           {
             { m } { \@@_print_meaning: \@@_prompt_aux: }
             { q }
@@ -7620,8 +7770,11 @@
               { \int_gset:Nn \g_@@_noise_int } }
             { C }
               {
-                \tl_gset_rescan:Nnx \g_@@_tmpc_tl
-                  { \ExplSyntaxOn } { \tl_tail:n {#1} }
+                \@@_exp_args:Nx \use:n
+                  {
+                    \tl_gset_rescan:Nnn \exp_not:N \g_@@_tmpc_tl
+                      { \exp_not:N \ExplSyntaxOn } { \tl_tail:n {#1} }
+                  }
                 \tl_gput_left:Nn \g_@@_tmpc_tl
                   { \tl_gclear:N \g_@@_tmpc_tl }
                 \group_insert_after:N \g_@@_tmpc_tl
@@ -7683,8 +7836,8 @@
     \tl_gset:Nf \g_@@_before_print_state_tl
       {
         \exp_args:NNf \exp_stop_f: \int_compare:nNnTF
-          { \int_eval:n { \seq_count:N \g_@@_prev_input_seq - #2 } }
-          #1 { \seq_count:N \g_@@_prev_input_seq }
+          { \int_eval:n { \@@_prev_input_count: - #2 } }
+          #1 { \@@_prev_input_count: }
           {
             \int_gset:Nn \g_@@_nonstop_int
               { \int_max:nn { \g_@@_nonstop_int } { 2 } }
@@ -7720,6 +7873,33 @@
 %
 % \subsubsection{Errors}
 %
+% \begin{macro}{\@@_not_implemented:n}
+%    \begin{macrocode}
+\cs_new_protected:Npn \@@_not_implemented:n #1
+  { \@@_error:nnnnn { not-implemented } {#1} { } { } { } }
+%    \end{macrocode}
+% \end{macro}
+%
+% \begin{macro}[int]{\@@_error:nnnnn, \@@_error:nxxxx}
+%   Errors within a group to make sure that none of the \pkg{l3msg}
+%   variables (or others) that may be currently in use in the code being
+%   debugged are modified.
+%    \begin{macrocode}
+\cs_new_protected:Npn \@@_error:nnnnn #1#2#3#4#5
+  {
+    \group_begin:
+    \msg_error:nnnnnn { unravel } {#1} {#2} {#3} {#4} {#5}
+    \group_end:
+  }
+\cs_new_protected:Npn \@@_error:nxxxx #1#2#3#4#5
+  {
+    \group_begin:
+    \msg_error:nnxxxx { unravel } {#1} {#2} {#3} {#4} {#5}
+    \group_end:
+  }
+%    \end{macrocode}
+% \end{macro}
+%
 % \begin{macro}[int]{\@@_tex_msg_new:nnn}
 %   This stores a \TeX{} error message.
 %    \begin{macrocode}
@@ -7737,10 +7917,12 @@
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_tex_error:nn #1#2
   {
+    \group_begin:
     \msg_error:nnxxx { unravel } { tex-error }
       { \tl_to_str:n {#2} }
       { \use:c { @@_tex_msg_error_#1: } }
       { \use:c { @@_tex_msg_help_#1: } }
+    \group_end:
   }
 \cs_generate_variant:Nn \@@_tex_error:nn { nV }
 %    \end{macrocode}
@@ -7753,10 +7935,11 @@
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_tex_fatal_error:nn #1#2
   {
-    \msg_error:nnxxx { unravel } { tex-fatal }
+    \@@_error:nxxxx { tex-fatal }
       { \tl_to_str:n {#2} }
       { \use:c { @@_tex_msg_error_#1: } }
       { \use:c { @@_tex_msg_help_#1: } }
+      { }
   }
 \cs_generate_variant:Nn \@@_tex_fatal_error:nn { nV }
 %    \end{macrocode}
@@ -7868,7 +8051,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_init_key_vars:}
+% \begin{macro}{\@@_init_key_vars:}
 %   Give variables that are affected by keys their default values (also
 %   controlled by keys).
 %    \begin{macrocode}
@@ -7885,7 +8068,7 @@
 %    \end{macrocode}
 % \end{macro}
 %
-% \begin{macro}[aux]{\@@_init_vars:}
+% \begin{macro}{\@@_init_vars:}
 %   Give initial values to variables used during the processing.  These
 %   have no reason to be modified by the user: neither directly nor
 %   through keys.
@@ -7921,7 +8104,7 @@
 % \end{macro}
 %
 % ^^A todo: improve error message
-% \begin{macro}[aux]{\@@_final_test:, \@@_final_bad:}
+% \begin{macro}{\@@_final_test:, \@@_final_bad:}
 %   Make sure that the \cs{unravel} finished correctly.  The error
 %   message is a bit primitive.
 %    \begin{macrocode}
@@ -7939,8 +8122,8 @@
   }
 \cs_new_protected_nopar:Npn \@@_final_bad:
   {
-    \msg_error:nnx { unravel } { internal }
-      { the-last-unravel-finished-badly }
+    \@@_error:nnnnn { internal }
+      { the-last-unravel-finished-badly } { } { } { }
   }
 %    \end{macrocode}
 % \end{macro}
@@ -7952,8 +8135,6 @@
   { Internal~error:~the~primitive~'#1'~is~not~known. }
 \msg_new:nnn { unravel } { extra-fi-or-else }
   { Extra~fi,~or,~or~else. }
-\msg_new:nnn { unravel } { missing-lbrace }
-  { Missing~left~brace~inserted. }
 \msg_new:nnn { unravel } { missing-dollar }
   { Missing~dollar~inserted. }
 \msg_new:nnn { unravel } { unknown-expandable }
@@ -7962,8 +8143,6 @@
   { Missing~font~identifier.~\iow_char:N\\nullfont~inserted. }
 \msg_new:nnn { unravel } { missing-rparen }
   { Missing~right~parenthesis~inserted~for~expression. }
-\msg_new:nnn { unravel } { missing-mudim }
-  { Missing~mu~unit. }
 \msg_new:nnn { unravel } { missing-cs }
   { Missing~control~sequence.~\iow_char:N\\inaccessible~inserted. }
 \msg_new:nnn { unravel } { missing-box }
@@ -7999,8 +8178,6 @@
   }
 \msg_new:nnn { unravel } { bad-unless }
   { \iow_char:N\\unless~not~followed~by~conditional. }
-\msg_new:nnn { unravel } { missing-endcsname }
-  { Missing~\iow_char:N\\endcsname~inserted. }
 \msg_new:nnn { unravel } { runaway-if }
   { Runaway~\iow_char:N\\if... }
 \msg_new:nnn { unravel } { runaway-macro-parameter }
@@ -8041,11 +8218,11 @@
 \@@_tex_msg_new:nnn { incompatible-mag }
   {
     Incompatible~magnification~
-    ( \int_to_arabic:n { \@@_mag: } ); \\
-    \ the~previous~value~will~be~retained
+    ( \int_to_arabic:n { \@@_mag: } );~
+    the~previous~value~will~be~retained
   }
   {
-    I~can~handle~only~one~magnification~ratio~per~job.~So~I've\\
+    I~can~handle~only~one~magnification~ratio~per~job.~So~I've~
     reverted~to~the~magnification~you~used~earlier~on~this~run.
   }
 \@@_tex_msg_new:nnn { illegal-mag }
@@ -8054,9 +8231,55 @@
     ( \int_to_arabic:n { \@@_mag: } )
   }
   { The~magnification~ratio~must~be~between~1~and~32768. }
+\@@_tex_msg_new:nnn { missing-number }
+  { Missing~number,~treated~as~zero }
+  {
+    A~number~should~have~been~here;~I~inserted~`0'.~
+    If~you~can't~figure~out~why~I~needed~to~see~a~number,~
+    look~up~`weird~error'~in~the~index~to~The~TeXbook.
+  }
+\@@_tex_msg_new:nnn { the-cannot }
+  { You~can't~use~`\tl_to_str:N\l_@@_head_tl'~after~\iow_char:N\\the }
+  { I'm~forgetting~what~you~said~and~using~zero~instead. }
 \@@_tex_msg_new:nnn { incompatible-units }
   { Incompatible~glue~units }
   { I'm~going~to~assume~that~1mu=1pt~when~they're~mixed. }
+\@@_tex_msg_new:nnn { missing-mu }
+  { Illegal~unit~of~measure~(mu~inserted) }
+  {
+    The~unit~of~measurement~in~math~glue~must~be~mu.~
+    To~recover~gracefully~from~this~error,~it's~best~to~
+    delete~the~erroneous~units;~e.g.,~type~`2'~to~delete~
+    two~letters.~(See~Chapter~27~of~The~TeXbook.)
+  }
+\@@_tex_msg_new:nnn { missing-pt }
+  { Illegal~unit~of~measure~(pt~inserted) }
+  {
+    Dimensions~can~be~in~units~of~em,~ex,~in,~pt,~pc,~
+    cm,~mm,~dd,~cc,~nd,~nc,~bp,~or~sp;~but~yours~is~a~new~one!~
+    I'll~assume~that~you~meant~to~say~pt,~for~printer's~points.~
+    To~recover~gracefully~from~this~error,~it's~best~to~
+    delete~the~erroneous~units;~e.g.,~type~`2'~to~delete~
+    two~letters.~(See~Chapter~27~of~The~TeXbook.)
+  }
+\@@_tex_msg_new:nnn { missing-lbrace }
+  { Missing~\iow_char:N\{~inserted }
+  {
+    A~left~brace~was~mandatory~here,~so~I've~put~one~in.~
+    You~might~want~to~delete~and/or~insert~some~corrections~
+    so~that~I~will~find~a~matching~right~brace~soon.~
+    (If~you're~confused~by~all~this,~try~typing~`I\iow_char:N\}'~now.)
+  }
+\@@_tex_msg_new:nnn { extra-endcsname }
+  { Extra~\token_to_str:c{endcsname} }
+  { I'm~ignoring~this,~since~I~wasn't~doing~a~\token_to_str:c{csname}. }
+\@@_tex_msg_new:nnn { missing-endcsname }
+  { Missing~\token_to_str:c{endcsname}~inserted }
+  {
+    The~control~sequence~marked~<to~be~read~again>~should~
+    not~appear~between~\token_to_str:c{csname}~and~
+    \token_to_str:c{endcsname}.
+  }
 %    \end{macrocode}
 %
 % Fatal \TeX{} error messages.
diff --git a/Master/texmf-dist/tex/latex/unravel/unravel.sty b/Master/texmf-dist/tex/latex/unravel/unravel.sty
index 9d051e10e64..941ccfb4109 100644
--- a/Master/texmf-dist/tex/latex/unravel/unravel.sty
+++ b/Master/texmf-dist/tex/latex/unravel/unravel.sty
@@ -11,7 +11,7 @@
 %% Communicate any suggestions for changing this package
 %% to Bruno Le Floch (blflatex@gmail.com).
 %% 
-%% File: unravel.dtx Copyright (C) 2013, 2015 Bruno Le Floch
+%% File: unravel.dtx Copyright (C) 2013, 2015, 2018 Bruno Le Floch
 %%
 %% It may be distributed and/or modified under the conditions of the
 %% LaTeX Project Public License (LPPL), either version 1.3c of this
@@ -146,14 +146,14 @@
     \newlinechar  = 10 ~
   }
 \expandafter \endgroup \__unravel_setup_latexe:
-\RequirePackage{expl3,xparse}[2015/09/11]
-\RequirePackage{gtl}[2015/09/21]
+\RequirePackage{expl3,xparse}[2018/02/21]
+\RequirePackage{gtl}[2018/04/04]
 \csname cs_if_exist:cF\endcsname{str_range:nnn}{\RequirePackage{l3str}}
 \csname use:n\endcsname
   {%
     \csname __unravel_setup_restore:\endcsname
     \ProvidesExplPackage
-      {unravel} {2015/09/30} {0.2} {Watching TeX digest tokens}%
+      {unravel} {2018/04/04} {0.2e} {Watching TeX digest tokens}%
     \csname __unravel_setup_unravel:\endcsname
   }%
 \cs_new_eq:NN \__unravel_currentgrouptype:      \etex_currentgrouptype:D
@@ -165,8 +165,10 @@
 \cs_new_eq:NN \__unravel_mag:                   \tex_mag:D
 \cs_new_eq:NN \__unravel_nullfont:              \tex_nullfont:D
 \cs_new_eq:NN \__unravel_the:w                  \tex_the:D
+\cs_new_eq:NN \__unravel_number:w               \tex_number:D
 \cs_new_eq:NN \c__unravel_prompt_ior \c_sixteen
 \cs_new_eq:NN \c__unravel_noprompt_ior \c_minus_one
+\cs_generate_variant:Nn \seq_push:Nn { Nf }
 \cs_generate_variant:Nn \str_head:n { f }
 \cs_generate_variant:Nn \tl_to_str:n { o }
 \cs_generate_variant:Nn \tl_if_head_eq_meaning:nNT { V }
@@ -174,9 +176,8 @@
 \cs_generate_variant:Nn \tl_if_in:nnTF { nV }
 \cs_generate_variant:Nn \tl_if_in:NnTF { No , NV }
 \cs_generate_variant:Nn \tl_if_single_token:nT { V }
-\cs_generate_variant:Nn \tl_gset_rescan:Nnn { Nnx }
 \cs_generate_variant:Nn \gtl_gput_right:Nn { NV }
-\cs_generate_variant:Nn \ior_get_str:NN { Nc }
+\cs_generate_variant:Nn \ior_str_get:NN { Nc }
 \cs_generate_variant:Nn \gtl_if_empty:NTF { c }
 \cs_generate_variant:Nn \gtl_to_str:N { c }
 \cs_generate_variant:Nn \gtl_gpop_left:NN { c }
@@ -185,6 +186,17 @@
 \cs_generate_variant:Nn \gtl_gconcat:NNN { ccc , cNc }
 \cs_generate_variant:Nn \gtl_gclear:N { c }
 \cs_generate_variant:Nn \gtl_gclear_new:N { c }
+\tl_new:N \l__unravel_exp_tl
+\cs_new_protected:Npn \__unravel_exp_args:Nx #1#2
+  {
+    \cs_set_nopar:Npx \l__unravel_exp_tl { \exp_not:N #1 {#2} }
+    \l__unravel_exp_tl
+  }
+\cs_new_protected:Npn \__unravel_exp_args:NNx #1#2#3
+  {
+    \cs_set_nopar:Npx \l__unravel_exp_tl { \exp_not:N #1 \exp_not:N #2 {#3} }
+    \l__unravel_exp_tl
+  }
 \cs_new_protected_nopar:Npn \__unravel_tmp:w { }
 \cs_set_protected:Npn \__unravel_tmp:w #1
   {
@@ -254,7 +266,7 @@
   }
 \cs_new:Npn \__unravel_strip_escape_aux:N #1 { \c_zero }
 \cs_new:Npn \__unravel_strip_escape_aux:w #1#2
-  { - \__int_value:w #1 \c_zero }
+  { - \__unravel_number:w #1 \c_zero }
 \cs_new:Npn \__unravel_to_str:n #1
   {
     \tl_if_head_eq_meaning:nNTF {#1} \scan_stop:
@@ -272,8 +284,8 @@
     \cs_new:Npn \__unravel_to_str_auxii:w ##1 #1 ##2 \q_mark ##3 \q_stop
       { \cs_if_exist_use:cF { __unravel_ ##2 _to_str:n } { \tl_to_str:n } }
   }
-\exp_args:No \__unravel_tmp:w { \tl_to_str:n { s__ } }
-\cs_new:Npn \__unravel_gtl_to_str:n #1 { \__gtl_to_str:w #1 }
+\exp_args:No \__unravel_tmp:w { \tl_to_str:n { s _ _ } }
+\cs_new:Npn \__unravel_gtl_to_str:n { \gtl_to_str:n }
 \cs_new:Npn \__unravel_str_truncate_left:nn #1#2
   {
     \exp_args:Nf \__unravel_str_truncate_left_aux:nnn
@@ -379,8 +391,7 @@
       {
         \gtl_if_head_is_N_type:NTF #1
           {
-            \exp_last_unbraced:Nx \__unravel_token_if_definable:NTF
-              { \gtl_head:N #1 }
+            \gtl_head_do:NN #1 \__unravel_token_if_definable:NTF
               { \prg_return_true: }
               { \prg_return_false: }
           }
@@ -388,24 +399,53 @@
       }
       { \prg_return_false: }
   }
+\cs_new_nopar:Npn \__unravel_prev_input_count:
+  {
+    \int_eval:n
+      {
+        0
+        \seq_map_function:NN \g__unravel_prev_input_seq
+          \__unravel_prev_input_count_aux:n
+      }
+  }
+\cs_new:Npn \__unravel_prev_input_count_aux:n #1
+  { \tl_if_empty:nF {#1} { + 1 } }
+\cs_new_protected_nopar:Npn \__unravel_prev_input_get:N
+  { \seq_get_right:NN \g__unravel_prev_input_seq }
+\cs_new_protected_nopar:Npn \__unravel_prev_input_gpush:
+  { \seq_gput_right:Nn \g__unravel_prev_input_seq { } }
+\cs_new_protected_nopar:Npn \__unravel_prev_input_gpush:N
+  { \seq_gput_right:NV \g__unravel_prev_input_seq }
+\cs_new_protected_nopar:Npn \__unravel_prev_input_gpop:N
+  { \seq_gpop_right:NN \g__unravel_prev_input_seq }
+\cs_new_protected_nopar:Npn \__unravel_prev_input_gpush_gtl:
+  { \seq_gput_right:NV \g__unravel_prev_input_seq \c_empty_gtl }
+\cs_new_protected_nopar:Npn \__unravel_prev_input_gpush_gtl:N
+  { \seq_gput_right:NV \g__unravel_prev_input_seq }
+\cs_new_protected_nopar:Npn \__unravel_prev_input_gpop_gtl:N
+  { \seq_gpop_right:NN \g__unravel_prev_input_seq }
 \cs_new_protected:Npn \__unravel_prev_input_silent:n #1
   {
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_prev_input_tl
+    \__unravel_prev_input_gpop:N \l__unravel_prev_input_tl
     \tl_put_right:Nn \l__unravel_prev_input_tl {#1}
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_prev_input_tl
+    \__unravel_prev_input_gpush:N \l__unravel_prev_input_tl
   }
-\cs_generate_variant:Nn \__unravel_prev_input_silent:n { V , x }
+\cs_generate_variant:Nn \__unravel_prev_input_silent:n { V }
+\cs_new_protected:Npn \__unravel_prev_input_silent:x
+  { \__unravel_exp_args:Nx \__unravel_prev_input_silent:n }
 \cs_new_protected:Npn \__unravel_prev_input:n #1
   {
     \__unravel_prev_input_silent:n {#1}
     \__unravel_print_action:x { \tl_to_str:n {#1} }
   }
-\cs_generate_variant:Nn \__unravel_prev_input:n { V , x }
+\cs_generate_variant:Nn \__unravel_prev_input:n { V }
+\cs_new_protected:Npn \__unravel_prev_input:x
+  { \__unravel_exp_args:Nx \__unravel_prev_input:n }
 \cs_new_protected:Npn \__unravel_prev_input_gtl:N #1
   {
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_prev_input_gtl
+    \__unravel_prev_input_gpop_gtl:N \l__unravel_prev_input_gtl
     \gtl_concat:NNN \l__unravel_prev_input_gtl \l__unravel_prev_input_gtl #1
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_prev_input_gtl
+    \__unravel_prev_input_gpush_gtl:N \l__unravel_prev_input_gtl
   }
 \cs_new_protected:Npn \__unravel_prev_input_join_get:nN #1
   {
@@ -415,14 +455,14 @@
         { 3 } { \__unravel_join_get_aux:NNN \muskip_eval:n \etex_muexpr:D }
       }
       {
-        \msg_error:nnn { unravel } { internal } { join-factor }
+        \__unravel_error:nnnnn { internal } { join-factor } { } { } { }
         \__unravel_join_get_aux:NNN \use:n \prg_do_nothing:
       }
   }
 \cs_new_protected:Npn \__unravel_join_get_aux:NNN #1#2#3
   {
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_tmpa_tl
+    \__unravel_prev_input_gpop:N \l__unravel_head_tl
+    \__unravel_prev_input_gpop:N \l__unravel_tmpa_tl
     \tl_set:Nx #3 { #1 { \l__unravel_tmpa_tl #2 \l__unravel_head_tl } }
   }
 \tl_new:N \g__unravel_before_print_state_tl
@@ -464,7 +504,7 @@
 \gtl_new:N \g__unravel_output_gtl
 \gtl_new:N \l__unravel_head_gtl
 \tl_new:N  \l__unravel_head_tl
-\token_new:Nn \l__unravel_head_token { ? }
+\cs_new_eq:NN \l__unravel_head_token ?
 \int_new:N \l__unravel_head_cmd_int
 \int_new:N \l__unravel_head_char_int
 \tl_new:N \l__unravel_head_meaning_tl
@@ -1273,8 +1313,8 @@
   {
     \exp_last_unbraced:NV \__unravel_set_cmd_aux_primitive:nn
       \c__unravel_tex_relax_tl
-    \msg_error:nnx { unravel } { unknown-primitive }
-      { \l__unravel_head_meaning_tl }
+    \__unravel_error:nxxxx { unknown-primitive }
+      { \l__unravel_head_meaning_tl } { } { } { }
   }
 \cs_new_protected_nopar:Npn \__unravel_set_cmd_aux_cs:
   {
@@ -1332,7 +1372,7 @@
       { \tl_set:Nn \l__unravel_head_meaning_tl { other~ } }
     \exp_after:wN \__unravel_set_cmd_aux_char:w
       \l__unravel_head_meaning_tl \q_stop
-    \exp_args:NNx \int_set:Nn \l__unravel_head_char_int
+    \__unravel_exp_args:NNx \int_set:Nn \l__unravel_head_char_int
       { ` \__unravel_token_to_char:N \l__unravel_head_token }
   }
 \cs_new_protected:Npn \__unravel_set_cmd_aux_char:w #1 ~ #2 \q_stop
@@ -1379,8 +1419,9 @@
         \gtl_if_head_is_N_type:NT #1
           {
             \tl_set:Nx \l__unravel_input_tmpa_tl { \gtl_left_tl:N #1 }
-            \exp_last_unbraced:Nx \__unravel_input_split_auxii:N
-              { \tl_head:N \l__unravel_input_tmpa_tl }
+            \__unravel_exp_args:NNx \use:nn
+                \__unravel_input_split_auxii:N
+                { \tl_head:N \l__unravel_input_tmpa_tl }
           }
       }
   }
@@ -1513,7 +1554,9 @@
     \gtl_gclear_new:c { g__unravel_input_ \int_use:N \g__unravel_input_int _gtl }
     \gtl_gset:cn { g__unravel_input_ \int_use:N \g__unravel_input_int _gtl }
   }
-\cs_generate_variant:Nn \__unravel_back_input:n { x , V , o }
+\cs_generate_variant:Nn \__unravel_back_input:n { V , o }
+\cs_new_protected:Npn \__unravel_back_input:x
+  { \__unravel_exp_args:Nx \__unravel_back_input:n }
 \cs_new_protected:Npn \__unravel_back_input_gtl:N #1
   {
     \gtl_if_tl:NTF #1
@@ -1544,17 +1587,18 @@
   }
 \cs_new_protected_nopar:Npn \__unravel_insert_group_begin_error:
   {
-    \msg_error:nn { unravel  } { missing-lbrace }
+    \tl_set_eq:NN \l__unravel_tmpa_tl \l__unravel_head_tl
     \__unravel_back_input:
     \gtl_set_eq:NN \l__unravel_head_gtl \c_group_begin_gtl
     \__unravel_back_input:
+    \__unravel_tex_error:nV { missing-lbrace } \l__unravel_tmpa_tl
     \__unravel_print_action:
   }
 \cs_new_protected_nopar:Npn \__unravel_insert_dollar_error:
   {
     \__unravel_back_input:
     \__unravel_back_input:n { $ } % $
-    \msg_error:nn { unravel } { missing-dollar }
+    \__unravel_error:nnnnn { missing-dollar } { } { } { } { }
     \__unravel_print_action:
   }
 \use:x
@@ -1612,8 +1656,8 @@
   }
 \cs_new_protected:Npn \__unravel_macro_call_quick_runaway:Nw #1#2 \q_stop
   {
-    \msg_error:nnxx { unravel } { runaway-macro-parameter }
-      { \tl_to_str:N \l__unravel_head_tl } { \tl_to_str:n {#1} }
+    \__unravel_error:nxxxx { runaway-macro-parameter }
+      { \tl_to_str:N \l__unravel_head_tl } { \tl_to_str:n {#1} } { } { }
   }
 \cs_new_protected_nopar:Npn \__unravel_expand:
   {
@@ -1621,7 +1665,7 @@
     \bool_if:NT \g__unravel_internal_debug_bool
       {
         \__unravel_set_cmd:
-        \iow_term:x { Exp:~\int_to_arabic:n { \l__unravel_head_cmd_int } }
+        \__unravel_exp_args:Nx \iow_term:n { Exp:~\int_to_arabic:n { \l__unravel_head_cmd_int } }
       }
     \token_if_macro:NTF \l__unravel_head_token
       { \__unravel_macro_call: }
@@ -1634,11 +1678,11 @@
       {
         \cs_if_exist_use:cF
           { __unravel_expandable_ \int_use:N \l__unravel_head_cmd_int : }
-          { \msg_error:nnx { unravel } { internal } { expandable } }
+          { \__unravel_error:nxxxx { internal } { expandable } { } { } { } }
       }
       {
-        \msg_error:nnx { unravel } { unknown-primitive }
-          { \l__unravel_head_meaning_tl }
+        \__unravel_error:nxxxx { unknown-primitive }
+          { \l__unravel_head_meaning_tl } { } { } { }
         \gtl_gput_right:NV \g__unravel_output_gtl \l__unravel_head_tl
         \__unravel_print_action:
       }
@@ -1697,14 +1741,17 @@
   {
     \__unravel_get_x_non_relax:
     \token_if_eq_catcode:NNF \l__unravel_head_token \c_group_begin_token
-      { \__unravel_insert_group_begin_error: }
+      {
+        \__unravel_insert_group_begin_error:
+        \__unravel_get_next:
+      }
   }
 \cs_new_protected:Npn \__unravel_scan_keyword:n #1
   { \__unravel_scan_keyword:nTF {#1} { } { } }
 \prg_new_protected_conditional:Npnn \__unravel_scan_keyword:n #1
   { T , F , TF }
   {
-    \seq_gput_right:NV \g__unravel_prev_input_seq \c_empty_gtl
+    \__unravel_prev_input_gpush_gtl:
     \__unravel_scan_keyword_loop:NNN \c_true_bool
       #1 \q_recursion_tail \q_recursion_tail \q_recursion_stop
   }
@@ -1713,7 +1760,7 @@
     \quark_if_recursion_tail_stop_do:nn {#2}
       { \__unravel_scan_keyword_true: }
     \quark_if_recursion_tail_stop_do:nn {#3}
-      { \msg_error:nnx { unravel } { internal } { odd-keyword-length } }
+      { \__unravel_error:nxxxx { internal } { odd-keyword-length } { } { } { } }
     \__unravel_get_x_next:
     \__unravel_scan_keyword_test:NNTF #2#3
       {
@@ -1747,7 +1794,7 @@
   }
 \cs_new_protected_nopar:Npn \__unravel_scan_keyword_true:
   {
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_tmpb_gtl
+    \__unravel_prev_input_gpop_gtl:N \l__unravel_tmpb_gtl
     \__unravel_prev_input:x { \gtl_to_str:N \l__unravel_tmpb_gtl }
     \prg_return_true:
   }
@@ -1755,7 +1802,7 @@
     #1 \q_recursion_stop
   {
     \__unravel_back_input:
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_tmpb_gtl
+    \__unravel_prev_input_gpop_gtl:N \l__unravel_tmpb_gtl
     \__unravel_back_input_gtl:N \l__unravel_tmpb_gtl
     \prg_return_false:
   }
@@ -1763,7 +1810,7 @@
   {
     \__unravel_scan_keyword:nF { tToO }
       {
-        \msg_error:nn { unravel } { missing-to }
+        \__unravel_error:nnnnn { missing-to } { } { } { } { }
         \__unravel_prev_input:n { to }
       }
   }
@@ -1788,7 +1835,7 @@
           }
       }
       {
-        \msg_error:nn { unravel } { missing-font-id }
+        \__unravel_error:nnnnn { missing-font-id } { } { } { } { }
         \__unravel_back_input:
         \__unravel_prev_input:n { \__unravel_nullfont: }
       }
@@ -1818,30 +1865,43 @@
   }
 \cs_new_protected:Npn \__unravel_scan_something_aux:nwn #1#2 \q_stop #3
   {
-    \int_compare:nNnTF
-      { ( #1 + \c_two ) / \c_four } > { ( #3 + \c_two ) / \c_four }
+    \int_compare:nT { #3 < 4 <= #1 }
       {
         \__unravel_back_input:
-        \msg_error:nn { unravel } { missing-something }
-        \tl_clear:N \l__unravel_tmpa_tl
+        \__unravel_tex_error:nV { missing-number } \l__unravel_head_tl
+        \__unravel_thing_use_get:nnNN { 1 } {#3} \c_zero_dim \l__unravel_tmpa_tl
+        \__unravel_scan_something_internal_auxii:Vn \l__unravel_tmpa_tl { 1 }
+        \__unravel_break:w
       }
+    \int_compare:nNnT {#1} = { 8 }
       {
-        \tl_if_empty:nF {#2}
-          {
-            \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
-            \__unravel_print_action:
-            #2
-            \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
-          }
-        \__unravel_thing_use_get:nnNN {#1} {#3} \l__unravel_head_tl \l__unravel_tmpa_tl
+        \__unravel_tex_error:nV { the-cannot } \l__unravel_head_tl
+        \__unravel_scan_something_internal_auxii:Vn \c_zero { 0 }
+        \__unravel_break:w
       }
-    \__unravel_prev_input_silent:V \l__unravel_tmpa_tl
+    \tl_if_empty:nF {#2}
+      {
+        \__unravel_prev_input_gpush:N \l__unravel_head_tl
+        \__unravel_print_action:
+        #2
+        \__unravel_prev_input_gpop:N \l__unravel_head_tl
+      }
+    \int_compare:nNnTF {#3} < { 4 }
+      { \__unravel_thing_use_get:nnNN {#1} {#3} \l__unravel_head_tl \l__unravel_tmpa_tl }
+      { \tl_set:Nx \l__unravel_tmpa_tl { \__unravel_the:w \l__unravel_head_tl } }
+    \__unravel_scan_something_internal_auxii:Vn \l__unravel_tmpa_tl {#1}
+    \__unravel_break_point:
+    \int_compare:nNnT {#3} < { 4 } { \__unravel_print_action: }
+  }
+\cs_new_protected:Npn \__unravel_scan_something_internal_auxii:nn #1#2
+  {
+    \__unravel_prev_input_silent:n {#1}
     \__unravel_set_action_text:
     \__unravel_set_action_text:x
-      { \g__unravel_action_text_str \use:n { ~ => ~ } \tl_to_str:N \l__unravel_tmpa_tl }
-    \int_compare:nNnF {#3} > { 3 } { \__unravel_print_action: }
-    \int_gset:Nn \g__unravel_val_level_int {#1}
+      { \g__unravel_action_text_str \use:n { ~ => ~ } \tl_to_str:n {#1} }
+    \int_gset:Nn \g__unravel_val_level_int {#2}
   }
+\cs_generate_variant:Nn \__unravel_scan_something_internal_auxii:nn { V }
 \cs_new_nopar:Npn \__unravel_thing_case:
   {
     \int_case:nnF \l__unravel_head_cmd_int
@@ -1868,6 +1928,8 @@
         { 87 } { 4                         } % set_font
         { 88 } { 4                         } % def_font
         { 89 } { \__unravel_thing_register:       } % register
+        {101 } { 4                         } % letterspace_font
+        {102 } { 4                         } % pdf_copy_font
       }
       { 8 }
   }
@@ -1945,19 +2007,15 @@
           }
       }
       {
-        \int_compare:nNnTF {#2} = { 3 }
+        \int_case:nnF {#1}
           {
-            \int_case:nnF {#1}
-              {
-                { 0 } { \tl_set:Nx #4 { \int_eval:n {#3} } }
-                { 3 } { \tl_set:Nx #4 { \muskip_eval:n {#3} } }
-              }
-              {
-                \__unravel_tex_error:nV { incompatible-units } #3
-                \tl_set:Nx #4 { \muskip_eval:n { \etex_gluetomu:D #3 } }
-              }
+            { 0 } { \tl_set:Nx #4 { \int_eval:n {#3} } }
+            { 3 } { \tl_set:Nx #4 { \muskip_eval:n {#3} } }
+          }
+          {
+            \__unravel_tex_error:nV { incompatible-units } #3
+            \tl_set:Nx #4 { \muskip_eval:n { \etex_gluetomu:D #3 } }
           }
-          { \tl_set:Nx #4 { \__unravel_the:w #3 } }
       }
   }
 \cs_new_protected:Npn \__unravel_scan_expr:N #1
@@ -2007,7 +2065,7 @@
       {
         \bool_if:NTF #2
           {
-            \msg_error:nn { unravel } { missing-rparen }
+            \__unravel_error:nnnnn { missing-rparen } { } { } { } { }
             \__unravel_back_input:
             \__unravel_prev_input:V \c__unravel_rp_tl
           }
@@ -2062,16 +2120,16 @@
           {
             \__unravel_prev_input:V \l__unravel_head_tl
             \__unravel_get_x_next:
-            \__unravel_scan_int_explicit:n { ' }
+            \__unravel_scan_int_explicit:Nn \c_false_bool { ' }
           }
         \c__unravel_dq_tl
           {
             \__unravel_prev_input:V \l__unravel_head_tl
             \__unravel_get_x_next:
-            \__unravel_scan_int_explicit:n { " }
+            \__unravel_scan_int_explicit:Nn \c_false_bool { " }
           }
       }
-      { \__unravel_scan_int_explicit:n { } }
+      { \__unravel_scan_int_explicit:Nn \c_false_bool { } }
   }
 \cs_new_protected_nopar:Npn \__unravel_scan_int_lq:
   {
@@ -2088,10 +2146,10 @@
       { ` \gtl_to_str:N \l__unravel_head_gtl = \l__unravel_tmpa_tl }
     \__unravel_skip_optional_space:
   }
-\cs_new_protected:Npn \__unravel_scan_int_explicit:n #1
+\cs_new_protected:Npn \__unravel_scan_int_explicit:Nn #1#2
   {
     \if_int_compare:w \c_one
-        < #1 1 \exp_after:wN \exp_not:N \l__unravel_head_tl \exp_stop_f:
+        < #2 1 \exp_after:wN \exp_not:N \l__unravel_head_tl \exp_stop_f:
       \exp_after:wN \use_i:nn
     \else:
       \exp_after:wN \use_ii:nn
@@ -2099,11 +2157,16 @@
     {
       \__unravel_prev_input:V \l__unravel_head_tl
       \__unravel_get_x_next:
-      \__unravel_scan_int_explicit:n {#1}
+      \__unravel_scan_int_explicit:Nn \c_true_bool {#2}
     }
     {
       \token_if_eq_catcode:NNF \l__unravel_head_token \c_space_token
         { \__unravel_back_input: }
+      \bool_if:NF #1
+        {
+          \__unravel_tex_error:nV { missing-number } \l__unravel_head_tl
+          \__unravel_prev_input:n { 0 }
+        }
     }
   }
 \cs_new_protected_nopar:Npn \__unravel_scan_normal_dimen:
@@ -2111,7 +2174,7 @@
 \cs_new_protected:Npn \__unravel_scan_dimen:nN #1#2
   {
     \__unravel_scan_signs:
-    \seq_gput_right:Nn \g__unravel_prev_input_seq { }
+    \__unravel_prev_input_gpush:
     \__unravel_set_cmd:
     \__unravel_cmd_if_internal:TF
       {
@@ -2122,7 +2185,7 @@
           { \__unravel_scan_dim_unit:nN {#1} #2 }
       }
       { \__unravel_scan_dimen_char:nN {#1} #2 }
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpop:N \l__unravel_head_tl
     \__unravel_prev_input_silent:V \l__unravel_head_tl
   }
 \cs_new_protected:Npn \__unravel_scan_dimen_char:nN #1#2
@@ -2169,17 +2232,18 @@
     \__unravel_set_cmd:
     \__unravel_cmd_if_internal:TF
       {
-        \seq_gput_right:Nn \g__unravel_prev_input_seq { }
+        \__unravel_prev_input_gpush:
         \__unravel_scan_something_internal:n {#1}
         \__unravel_prev_input_join_get:nN {#1} \l__unravel_tmpa_tl
-        \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_tmpa_tl
+        \__unravel_prev_input_gpush:N \l__unravel_tmpa_tl
         \__unravel_break:w
       }
       { \__unravel_back_input: }
     \int_compare:nNnT {#1} = { 3 }
       {
         \__unravel_scan_keyword:nT { mMuU } { \__unravel_break:w }
-        \msg_error:nn { unravel } { missing-mudim }
+        \__unravel_tex_error:nV { missing-mu } \l__unravel_head_tl
+        \__unravel_prev_input:n { mu }
         \__unravel_break:w
       }
     \__unravel_scan_keyword:nT { eEmM } { \__unravel_break:w }
@@ -2198,6 +2262,8 @@
     \__unravel_scan_keyword:nT { nNdD } { \__unravel_break:w }
     \__unravel_scan_keyword:nT { nNcC } { \__unravel_break:w }
     \__unravel_scan_keyword:nT { sSpP } { \__unravel_break:w }
+    \__unravel_tex_error:nV { missing-pt } \l__unravel_head_tl
+    \__unravel_prev_input:n { pt }
     \__unravel_break_point:
   }
 \cs_new_protected_nopar:Npn \__unravel_scan_inf_unit_loop:
@@ -2224,24 +2290,24 @@
   { \__unravel_scan_glue:n { 3 } }
 \cs_new_protected:Npn \__unravel_scan_glue:n #1
   {
-    \seq_gput_right:Nn \g__unravel_prev_input_seq { }
+    \__unravel_prev_input_gpush:
     \__unravel_scan_signs:
-    \seq_gput_right:Nn \g__unravel_prev_input_seq { }
+    \__unravel_prev_input_gpush:
     \__unravel_set_cmd:
     \__unravel_cmd_if_internal:TF
       {
         \__unravel_scan_something_internal:n {#1}
         \int_case:nnF \g__unravel_val_level_int
           {
-            { 0 } { \__unravel_scan_dimen:nN {#1} \c_false_bool }
+            { 0 } { \__unravel_scan_dim_unit:nN {#1} \c_false_bool }
             { 1 } { }
           }
           { \__unravel_break:w }
       }
       { \__unravel_back_input: \__unravel_scan_dimen:nN {#1} \c_false_bool }
     \__unravel_prev_input_join_get:nN {#1} \l__unravel_tmpa_tl
-    \seq_gput_right:Nn \g__unravel_prev_input_seq { }
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_tmpa_tl
+    \__unravel_prev_input_gpush:
+    \__unravel_prev_input_gpush:N \l__unravel_tmpa_tl
     \__unravel_scan_keyword:nT { pPlLuUsS }
       { \__unravel_scan_dimen:nN {#1} \c_true_bool }
     \__unravel_scan_keyword:nT { mMiInNuUsS }
@@ -2280,7 +2346,7 @@
       { \__unravel_get_next: }
     \__unravel_gtl_if_head_is_definable:NF \l__unravel_head_gtl
       {
-        \msg_error:nn { unravel } { missing-cs }
+        \__unravel_error:nnnnn { missing-cs } { } { } { } { }
         \__unravel_back_input:
         \tl_set:Nn \l__unravel_head_tl { \__unravel_inaccessible:w }
       }
@@ -2289,9 +2355,9 @@
   }
 \cs_new_protected:Npn \__unravel_scan_toks_to_str:
   {
-    \seq_gput_right:Nn \g__unravel_prev_input_seq { }
+    \__unravel_prev_input_gpush:
     \__unravel_scan_toks:NN \c_false_bool \c_true_bool
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_tmpa_tl
+    \__unravel_prev_input_gpop:N \l__unravel_tmpa_tl
     \__unravel_prev_input_silent:x
       { { \exp_after:wN \tl_to_str:n \l__unravel_tmpa_tl } }
   }
@@ -2323,7 +2389,7 @@
     \__unravel_back_input:
     \__unravel_input_gpop_item:NF \l__unravel_head_tl
       {
-        \msg_error:nn { unravel } { runaway-text }
+        \__unravel_error:nnnnn { runaway-text } { } { } { } { }
         \__unravel_exit:w
       }
     \tl_set:Nx \l__unravel_head_tl { { \exp_not:V \l__unravel_head_tl } }
@@ -2342,10 +2408,10 @@
         \__unravel_scan_group_xdef:n { 1 }
       }
       {
-        \seq_gput_right:NV \g__unravel_prev_input_seq \c_empty_gtl
+        \__unravel_prev_input_gpush_gtl:
         \__unravel_prev_input_gtl:N \l__unravel_head_gtl
         \__unravel_scan_group_x:n { 1 }
-        \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_tmpb_gtl
+        \__unravel_prev_input_gpop_gtl:N \l__unravel_tmpb_gtl
         \__unravel_prev_input_silent:x
           { \gtl_left_tl:N \l__unravel_tmpb_gtl }
       }
@@ -2408,7 +2474,7 @@
                 \__unravel_prev_input:V \l__unravel_head_tl
               }
               {
-                \gtl_set:Nx \l__unravel_tmpb_gtl { \l__unravel_head_tl }
+                \__unravel_exp_args:NNx \gtl_set:Nn \l__unravel_tmpb_gtl { \l__unravel_head_tl }
                 \__unravel_prev_input_gtl:N \l__unravel_tmpb_gtl
                 \__unravel_print_action:
               }
@@ -2472,8 +2538,8 @@
 \cs_new_protected_nopar:Npn \__unravel_do_box_error:
   {
     \__unravel_back_input:
-    \msg_error:nn { unravel } { missing-box }
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_error:nnnnn { missing-box } { } { } { } { }
+    \__unravel_prev_input_gpop:N \l__unravel_head_tl
     \__unravel_print_action:x { \tl_to_str:N \l__unravel_head_tl }
   }
 \cs_new_protected:Npn \__unravel_do_begin_box:N #1
@@ -2498,7 +2564,7 @@
   {
     \bool_if:NTF #1 { \__unravel_do_leaders_fetch_skip: }
       {
-        \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpop:N \l__unravel_head_tl
         \tl_if_head_eq_meaning:VNT \l__unravel_head_tl \tex_shipout:D
           { \__unravel_prepare_mag: }
         \tl_use:N \l__unravel_head_tl \scan_stop:
@@ -2513,14 +2579,14 @@
     \int_compare:nNnTF \l__unravel_head_cmd_int
       = { \__unravel_tex_use:n { \mode_if_vertical:TF { vskip } { hskip } } }
       {
-        \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_tmpa_tl
+        \__unravel_prev_input_gpop:N \l__unravel_tmpa_tl
         \tl_put_left:NV \l__unravel_head_tl \l__unravel_tmpa_tl
         \__unravel_do_append_glue:
       }
       {
         \__unravel_back_input:
-        \msg_error:nn { unravel } { improper-leaders }
-        \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_error:nnnnn { improper-leaders } { } { } { } { }
+        \__unravel_prev_input_gpop:N \l__unravel_head_tl
         \__unravel_print_action:x { \tl_to_str:N \l__unravel_head_tl }
       }
   }
@@ -2531,10 +2597,10 @@
       { \__unravel_box_hook:N \tex_everyvbox:D }
       % ^^A todo: TeX calls |normal_paragraph| here.
     \__unravel_scan_spec:
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpop:N \l__unravel_head_tl
     \__unravel_set_action_text:x
       { \tl_to_str:N \l__unravel_head_tl \iow_char:N \{ }
-    \seq_push:Nx \l__unravel_leaders_box_seq
+    \seq_push:Nf \l__unravel_leaders_box_seq
       { \bool_if:NTF #1 { \mode_if_vertical:TF { v } { h } } { Z } }
     \gtl_gput_right:NV \g__unravel_output_gtl \l__unravel_head_tl
     \gtl_gconcat:NNN \g__unravel_output_gtl
@@ -2548,7 +2614,7 @@
     \str_if_eq_x:nnF
       { \tl_head:N \l__unravel_tmpa_tl } { \exp_not:N \__unravel_box_hook:w }
       {
-        \exp_args:Nx #1
+        \__unravel_exp_args:Nx #1
           {
             \exp_not:n { \__unravel_box_hook:w \prg_do_nothing: }
             \exp_not:V #1
@@ -2598,18 +2664,22 @@
     \gtl_gput_right:NV \g__unravel_output_gtl \l__unravel_tmpa_tl
     \__unravel_print_action:x { \tl_to_str:N \l__unravel_tmpa_tl }
   }
-\cs_generate_variant:Nn \__unravel_char:n { V , x }
+\cs_generate_variant:Nn \__unravel_char:n { V }
+\cs_new_protected:Npn \__unravel_char:x
+  { \__unravel_exp_args:Nx \__unravel_char:n }
 \cs_new_protected:Npn \__unravel_char_in_mmode:n #1
   {
     \int_compare:nNnTF { \tex_mathcode:D #1 } = { "8000 }
       { % math active
-        \gtl_set:Nx \l__unravel_head_gtl
+        \__unravel_exp_args:NNx \gtl_set:Nn \l__unravel_head_gtl
           { \char_generate:nn {#1} { 12 } }
         \__unravel_back_input:
       }
       { \__unravel_char:n {#1} }
   }
-\cs_generate_variant:Nn \__unravel_char_in_mmode:n { V , x }
+\cs_generate_variant:Nn \__unravel_char_in_mmode:n { V }
+\cs_new_protected:Npn \__unravel_char_in_mmode:x
+  { \__unravel_exp_args:Nx \__unravel_char_in_mmode:n }
 \cs_new_protected:Npn \__unravel_mathchar:n #1
   {
     \tex_mathchar:D #1 \scan_stop:
@@ -2618,7 +2688,8 @@
     \gtl_gput_right:NV \g__unravel_output_gtl \l__unravel_tmpa_tl
     \__unravel_print_action:x { \tl_to_str:N \l__unravel_tmpa_tl }
   }
-\cs_generate_variant:Nn \__unravel_mathchar:n { x }
+\cs_new_protected:Npn \__unravel_mathchar:x
+  { \__unravel_exp_args:Nx \__unravel_mathchar:n }
 \cs_new_protected:Npn \__unravel_new_graf:N #1
   {
     \tl_set:NV \l__unravel_tmpa_tl \__unravel_everypar:w
@@ -2706,8 +2777,8 @@
   {
     \int_compare:nNnTF \__unravel_currentgrouptype: = { 0 }
       { % bottom-level
-        \msg_error:nnx { unravel } { extra-close }
-          { \token_to_meaning:N \l__unravel_head_token }
+        \__unravel_error:nxxxx { extra-close }
+          { \token_to_meaning:N \l__unravel_head_token } { } { } { }
       }
       {
         \__unravel_back_input:
@@ -2722,8 +2793,8 @@
           }
           { \gtl_set_eq:NN \l__unravel_head_gtl \c_group_end_gtl }
         \__unravel_back_input:
-        \msg_error:nnx { unravel } { off-save }
-          { \gtl_to_str:N \l__unravel_head_gtl }
+        \__unravel_error:nxxxx { off-save }
+          { \gtl_to_str:N \l__unravel_head_gtl } { } { } { }
       }
   }
 \cs_new_protected:Npn \__unravel_mode_math:n #1
@@ -2748,7 +2819,7 @@
       {
         \token_if_eq_meaning:NNTF \l__unravel_head_token \tex_hrule:D
           {
-            \msg_error:nn { unravel } { hrule-bad-mode }
+            \__unravel_error:nnnnn { hrule-bad-mode } { } { } { } { }
             \__unravel_print_action:
           }
           { \__unravel_off_save: }
@@ -2787,7 +2858,7 @@
           {
             \__unravel_back_input:
             \tl_set:Nn \l__unravel_head_tl { $ } % $
-            \msg_error:nn { unravel } { missing-dollar }
+            \__unravel_error:nnnnn { missing-dollar } { } { } { } { }
           }
         \gtl_gput_right:NV \g__unravel_output_gtl \l__unravel_head_tl
         \__unravel_back_input_gtl:N \l__unravel_after_group_gtl
@@ -2799,10 +2870,10 @@
   {
     \__unravel_set_action_text:
     \bool_if:NT \g__unravel_internal_debug_bool
-      { \iow_term:x { Cmd:~\int_to_arabic:n { \l__unravel_head_cmd_int } } }
+      { \__unravel_exp_args:Nx \iow_term:n { Cmd:~\int_to_arabic:n { \l__unravel_head_cmd_int } } }
     \cs_if_exist_use:cF
       { __unravel_cmd_ \int_use:N \l__unravel_head_cmd_int : }
-      { \msg_error:nnx { unravel } { internal } { unknown-command } }
+      { \__unravel_error:nxxxx { internal } { unknown-command } { } { } { } }
   }
 \__unravel_new_tex_cmd:nn { relax }                                % 0
   { \__unravel_print_action: }
@@ -2857,7 +2928,7 @@
   { \__unravel_mode_math:n { \__unravel_sub_sup: } }
 \cs_new_protected_nopar:Npn \__unravel_sub_sup:
   {
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpush:N \l__unravel_head_tl
     \__unravel_print_action:
     \__unravel_get_x_non_relax:
     \__unravel_set_cmd:
@@ -2885,14 +2956,14 @@
           { \__unravel_prev_input:V \l__unravel_head_tl \__unravel_scan_int: }
       }
       {
-        \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpop:N \l__unravel_head_tl
         \gtl_gput_right:NV \g__unravel_output_gtl \l__unravel_head_tl
         \tl_use:N \l__unravel_head_tl \scan_stop:
       }
       {
         \__unravel_back_input:
         \__unravel_scan_left_brace:
-        \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpop:N \l__unravel_head_tl
         \gtl_gput_right:NV \g__unravel_output_gtl \l__unravel_head_tl
         \gtl_gconcat:NNN \g__unravel_output_gtl
           \g__unravel_output_gtl \c_group_begin_gtl
@@ -2901,7 +2972,7 @@
     \__unravel_print_action:x { \tl_to_str:N \l__unravel_head_tl }
   }
 \__unravel_new_tex_cmd:nn { endv }                                 % 9
-  { \msg_error:nn { unravel } { not-implemented } { alignments } }
+  { \__unravel_not_implemented:n { alignments } }
 \__unravel_new_tex_cmd:nn { blank_char }                           % 10
   {
     \mode_if_horizontal:T
@@ -2969,10 +3040,10 @@
   {
     \__unravel_mode_math:n
       {
-        \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpush:N \l__unravel_head_tl
         \__unravel_print_action:
         \__unravel_scan_int:
-        \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpop:N \l__unravel_head_tl
         \tl_use:N \l__unravel_head_tl \scan_stop:
         \__unravel_print_action:x { \tl_to_str:N \l__unravel_head_tl }
       }
@@ -2981,10 +3052,10 @@
   {
     \__unravel_mode_non_vertical:n
       {
-        \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpush:N \l__unravel_head_tl
         \__unravel_print_action:
         \__unravel_scan_int:
-        \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpop:N \l__unravel_head_tl
         \mode_if_math:TF
           { \__unravel_char_in_mmode:x { \tl_tail:N \l__unravel_head_tl } }
           { \__unravel_char:x { \tl_tail:N \l__unravel_head_tl } }
@@ -2994,23 +3065,23 @@
   {
     \__unravel_mode_math:n
       {
-        \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpush:N \l__unravel_head_tl
         \__unravel_print_action:
         \__unravel_scan_int:
-        \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpop:N \l__unravel_head_tl
         \__unravel_mathchar:x { \tl_tail:N \l__unravel_head_tl }
       }
   }
 \__unravel_new_tex_cmd:nn { mark }                                 % 18
   {
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpush:N \l__unravel_head_tl
     \__unravel_print_action:
     \int_compare:nNnF \l__unravel_head_char_int = \c_zero
       { \__unravel_scan_int: }
-    \seq_gput_right:Nn \g__unravel_prev_input_seq { }
+    \__unravel_prev_input_gpush:
     \__unravel_scan_toks:NN \c_false_bool \c_true_bool
-    \seq_gpop_right:Nn \g__unravel_prev_input_seq \l__unravel_tmpa_tl
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpop:N \l__unravel_tmpa_tl
+    \__unravel_prev_input_gpop:N \l__unravel_head_tl
     \__unravel_print_action:x
       { \tl_to_str:N \l__unravel_head_tl \tl_to_str:N \l__unravel_tmpa_tl }
     \tl_put_right:Nx \l__unravel_head_tl
@@ -3019,23 +3090,23 @@
   }
 \__unravel_new_tex_cmd:nn { xray }                                 % 19
   {
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpush:N \l__unravel_head_tl
     \__unravel_print_action:
     \int_case:nnF \l__unravel_head_char_int
       {
         { 0 }
           { % show
             \__unravel_get_next:
-            \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_tmpa_tl
+            \__unravel_prev_input_gpop:N \l__unravel_tmpa_tl
             \gtl_head_do:NN \l__unravel_head_gtl \l__unravel_tmpa_tl
           }
         { 2 }
           { % showthe
             \__unravel_get_x_next:
             \__unravel_scan_something_internal:n { 5 }
-            \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
-            \exp_args:Nx \etex_showtokens:D
-              { \tl_tail:N \l__unravel_head_tl }
+            \__unravel_prev_input_gpop:N \l__unravel_head_tl
+            \__unravel_exp_args:Nx \use:n
+              { \etex_showtokens:D { \tl_tail:N \l__unravel_head_tl } }
           }
       }
       { % no operand for showlists, showgroups, showifs
@@ -3043,19 +3114,19 @@
           { \__unravel_scan_int: }
         \int_compare:nNnT \l__unravel_head_char_int = \c_five % showtokens
           { \__unravel_scan_toks:NN \c_false_bool \c_false_bool }
-        \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpop:N \l__unravel_head_tl
         \tl_use:N \l__unravel_head_tl \scan_stop:
       }
   }
 \__unravel_new_tex_cmd:nn { make_box }                             % 20
   {
-    \seq_gput_right:Nn \g__unravel_prev_input_seq { }
+    \__unravel_prev_input_gpush:
     \__unravel_back_input:
     \__unravel_do_box:N \c_false_bool
   }
 \cs_new_protected_nopar:Npn \__unravel_do_move:
   {
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpush:N \l__unravel_head_tl
     \__unravel_print_action:
     \__unravel_scan_normal_dimen:
     \__unravel_do_box:N \c_false_bool
@@ -3072,10 +3143,10 @@
   }
 \cs_new_protected_nopar:Npn \__unravel_do_unpackage:
   {
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpush:N \l__unravel_head_tl
     \__unravel_print_action:
     \__unravel_scan_int:
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpop:N \l__unravel_head_tl
     \tl_use:N \l__unravel_head_tl \scan_stop:
     \__unravel_print_action:x { \tl_to_str:N \l__unravel_head_tl }
   }
@@ -3097,11 +3168,11 @@
     \int_compare:nNnTF \l__unravel_head_char_int < { 4 }
       { \tl_use:N \l__unravel_head_tl \__unravel_print_action: }
       {
-        \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpush:N \l__unravel_head_tl
         \__unravel_print_action:
         \exp_args:Nf \__unravel_scan_glue:n
           { \int_eval:n { \l__unravel_head_char_int - 2 } }
-        \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpop:N \l__unravel_head_tl
         \tl_use:N \l__unravel_head_tl \scan_stop:
         \__unravel_print_action:x { \tl_to_str:N \l__unravel_head_tl }
       }
@@ -3114,12 +3185,12 @@
   { \__unravel_mode_math:n { \__unravel_do_append_glue: } }
 \cs_new_protected_nopar:Npn \__unravel_do_append_kern:
   {
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpush:N \l__unravel_head_tl
     \__unravel_print_action:
     \token_if_eq_meaning:NNTF \l__unravel_head_token \tex_kern:D
       { \__unravel_scan_dimen:nN { 2 } \c_false_bool }
       { \__unravel_scan_dimen:nN { 3 } \c_false_bool }
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpop:N \l__unravel_head_tl
     \tl_use:N \l__unravel_head_tl \scan_stop:
     \__unravel_print_action:x { \tl_to_str:N \l__unravel_head_tl }
   }
@@ -3129,26 +3200,26 @@
   { \__unravel_mode_math:n { \__unravel_do_append_kern: } }
 \__unravel_new_tex_cmd:nn { leader_ship }                          % 31
   {
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpush:N \l__unravel_head_tl
     \__unravel_print_action:
     \__unravel_do_box:N \c_true_bool
   }
 \__unravel_new_tex_cmd:nn { halign }                               % 32
-  { \msg_fatal:nnx { unravel } { not-implemented } { halign } }
+  { \__unravel_not_implemented:n { halign } }
 \__unravel_new_tex_cmd:nn { valign }                               % 33
-  { \msg_fatal:nnx { unravel } { not-implemented } { valign } }
+  { \__unravel_not_implemented:n { valign } }
 \__unravel_new_tex_cmd:nn { no_align }                             % 34
-  { \msg_fatal:nnx { unravel } { not-implemented } { noalign } }
+  { \__unravel_not_implemented:n { noalign } }
 \__unravel_new_tex_cmd:nn { vrule }                                % 35
   { \__unravel_mode_non_vertical:n { \__unravel_do_rule: } }
 \__unravel_new_tex_cmd:nn { hrule }                                % 36
   { \__unravel_mode_vertical:n { \__unravel_do_rule: } }
 \cs_new_protected_nopar:Npn \__unravel_do_rule:
   {
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpush:N \l__unravel_head_tl
     \__unravel_print_action:
     \__unravel_scan_alt_rule:
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpop:N \l__unravel_head_tl
     \tl_use:N \l__unravel_head_tl \scan_stop:
     \__unravel_print_action:x { \tl_to_str:N \l__unravel_head_tl }
   }
@@ -3163,11 +3234,12 @@
   {
     \token_if_eq_meaning:NNTF \l__unravel_head_token \tex_ignorespaces:D
       {
+        \__unravel_print_action:
         \__unravel_get_x_non_blank:
         \__unravel_set_cmd:
         \__unravel_do_step:
       }
-      { \msg_error:nn { unravel } { not-implemented } { pdfprimitive } }
+      { \__unravel_not_implemented:n { pdfprimitive } }
   }
 \__unravel_new_tex_cmd:nn { after_assignment }                     % 40
   {
@@ -3205,10 +3277,10 @@
   }
 \__unravel_new_tex_cmd:nn { break_penalty }                        % 42
   {
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpush:N \l__unravel_head_tl
     \__unravel_print_action:
     \__unravel_scan_int:
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpop:N \l__unravel_head_tl
     \tl_use:N \l__unravel_head_tl \scan_stop:
     \__unravel_print_action:x { \tl_to_str:N \l__unravel_head_tl }
   }
@@ -3236,7 +3308,7 @@
   }
 \cs_new_protected_nopar:Npn \__unravel_do_accent:
   {
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpush:N \l__unravel_head_tl
     \__unravel_print_action:
     \__unravel_scan_int:
     \__unravel_do_assignments:
@@ -3260,7 +3332,7 @@
           }
           { \__unravel_break:w }
       }
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpop:N \l__unravel_head_tl
     \gtl_gput_right:NV \g__unravel_output_gtl \l__unravel_head_tl
     \tl_use:N \l__unravel_head_tl \scan_stop:
     \__unravel_print_action:x { \tl_to_str:N \l__unravel_head_tl }
@@ -3268,11 +3340,11 @@
   }
 \cs_new_protected_nopar:Npn \__unravel_do_math_accent:
   {
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpush:N \l__unravel_head_tl
     \__unravel_print_action:
     \__unravel_scan_int:
     \__unravel_scan_math:
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpop:N \l__unravel_head_tl
     \gtl_gput_right:NV \g__unravel_output_gtl \l__unravel_head_tl
     \tl_use:N \l__unravel_head_tl \scan_stop:
     \__unravel_print_action:x { \tl_to_str:N \l__unravel_head_tl }
@@ -3288,30 +3360,30 @@
 \__unravel_new_tex_cmd:nn { math_accent }                          % 46
   { \__unravel_mode_math:n { \__unravel_do_math_accent: } }
 \__unravel_new_tex_cmd:nn { discretionary }                        % 47
-  { \msg_error:nnx { unravel } { not-implemented } { discretionary } }
+  { \__unravel_not_implemented:n { discretionary } }
 \__unravel_new_tex_cmd:nn { eq_no }                                % 48
-  { \msg_error:nnx { unravel } { not-implemented } { eqno } }
+  { \__unravel_not_implemented:n { eqno } }
 \__unravel_new_tex_cmd:nn { left_right }                           % 49
-  { \msg_error:nnx { unravel } { not-implemented } { left/right } }
+  { \__unravel_not_implemented:n { left/right } }
 \__unravel_new_tex_cmd:nn { math_comp }                            % 50
-  { \msg_error:nnx { unravel } { not-implemented } { math~comp } }
+  { \__unravel_not_implemented:n { math~comp } }
 \__unravel_new_tex_cmd:nn { limit_switch }                         % 51
-  { \msg_error:nnx { unravel } { not-implemented } { limits } }
+  { \__unravel_not_implemented:n { limits } }
 \__unravel_new_tex_cmd:nn { above }                                % 52
-  { \msg_error:nnx { unravel } { not-implemented } { above } }
+  { \__unravel_not_implemented:n { above } }
 \__unravel_new_tex_cmd:nn { math_style }                           % 53
-  { \msg_error:nnx { unravel } { not-implemented } { math~style } }
+  { \__unravel_not_implemented:n { math~style } }
 \__unravel_new_tex_cmd:nn { math_choice }                          % 54
-  { \msg_error:nnx { unravel } { not-implemented } { math~choice } }
+  { \__unravel_not_implemented:n { math~choice } }
 \__unravel_new_tex_cmd:nn { non_script }                           % 55
-  { \msg_error:nnx { unravel } { not-implemented } { non~script } }
+  { \__unravel_not_implemented:n { non~script } }
 \__unravel_new_tex_cmd:nn { vcenter }                              % 56
-  { \msg_error:nnx { unravel } { not-implemented } { vcenter } }
+  { \__unravel_not_implemented:n { vcenter } }
 \__unravel_new_tex_cmd:nn { case_shift }                           % 57
   {
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpush:N \l__unravel_head_tl
     \__unravel_scan_toks:NN \c_false_bool \c_false_bool
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_tmpa_tl
+    \__unravel_prev_input_gpop:N \l__unravel_tmpa_tl
     \exp_after:wN \__unravel_case_shift:Nn \l__unravel_tmpa_tl
   }
 \cs_new_protected:Npn \__unravel_case_shift:Nn #1#2
@@ -3322,25 +3394,25 @@
   }
 \__unravel_new_tex_cmd:nn { message }                              % 58
   {
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpush:N \l__unravel_head_tl
     \__unravel_print_action:
     \__unravel_scan_toks_to_str:
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpop:N \l__unravel_head_tl
     \tl_use:N \l__unravel_head_tl
     \__unravel_print_action:x { \tl_to_str:N \l__unravel_head_tl }
   }
 \__unravel_new_tex_cmd:nn { extension }                            % 59
   {
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpush:N \l__unravel_head_tl
     \__unravel_print_action:
     \__unravel_scan_extension_operands:
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpop:N \l__unravel_head_tl
     \tl_use:N \l__unravel_head_tl \scan_stop:
     \__unravel_print_action:x { \tl_to_str:N \l__unravel_head_tl }
   }
 \__unravel_new_tex_cmd:nn { in_stream }                            % 60
   {
-    \seq_put_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpush:N \l__unravel_head_tl
     \__unravel_print_action:
     \token_if_eq_meaning:NNTF \l__unravel_head_token \tex_openin:D
       {
@@ -3349,7 +3421,7 @@
         \__unravel_scan_file_name:
       }
       { \__unravel_scan_int: }
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpop:N \l__unravel_head_tl
     \tl_use:N \l__unravel_head_tl \scan_stop:
     \__unravel_print_action:x { \tl_to_str:N \l__unravel_head_tl }
   }
@@ -3368,7 +3440,7 @@
     \__unravel_print_action:
   }
 \__unravel_new_tex_cmd:nn { omit }                                 % 63
-  { \msg_error:nn { unravel } { not-implemented } { omit } }
+  { \__unravel_not_implemented:n { omit } }
 \__unravel_new_tex_cmd:nn { ex_space }                             % 64
   {
     \__unravel_mode_non_vertical:n
@@ -3383,18 +3455,21 @@
   {
     \__unravel_mode_math:n
       {
-        \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpush:N \l__unravel_head_tl
         \__unravel_print_action:
         \__unravel_scan_int:
         \__unravel_scan_math:
-        \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpop:N \l__unravel_head_tl
         \gtl_gput_right:NV \g__unravel_output_gtl \l__unravel_head_tl
         \tl_use:N \l__unravel_head_tl \scan_stop:
         \__unravel_print_action:x { \tl_to_str:N \l__unravel_head_tl }
       }
   }
 \__unravel_new_tex_cmd:nn { end_cs_name }                          % 67
-  { \l__unravel_head_token \__unravel_print_action: }
+  {
+    \__unravel_tex_error:nV { extra-endcsname } \l__unravel_head_tl
+    \__unravel_print_action:
+  }
 \__unravel_new_tex_cmd:nn { char_given }                           % 68
   {
     \__unravel_mode_non_vertical:n
@@ -3436,7 +3511,7 @@
           {
             \mode_if_horizontal:TF
               { \__unravel_scan_int: }
-              { \msg_error:nn { unravel } { invalid-mode } }
+              { \__unravel_error:nnnnn { invalid-mode } { } { } { } { } }
           }
         { 6 } % pdfliteral
           {
@@ -3492,7 +3567,7 @@
         { 14 } % pdfstartlink
           {
             \mode_if_vertical:TF
-              { \msg_error:nn { unravel } { invalid-mode } }
+              { \__unravel_error:nnnnn { invalid-mode } { } { } { } { } }
               {
                 \__unravel_scan_rule_attr:
                 \__unravel_scan_action:
@@ -3501,7 +3576,7 @@
         { 15 } % pdfendlink
           {
             \mode_if_vertical:T
-              { \msg_error:nn { unravel } { invalid-mode } }
+              { \__unravel_error:nnnnn { invalid-mode } { } { } { } { } }
           }
         { 16 } % pdfoutline
           {
@@ -3597,8 +3672,8 @@
               {
                 \__unravel_scan_keyword:nF { cCuUrRrReEnNtT }
                   {
-                    \msg_error:nn { unravel }
-                      { color-stack-action-missing }
+                    \__unravel_error:nnnnn { color-stack-action-missing }
+                      { } { } { } { }
                   }
               }
           }
@@ -3618,7 +3693,7 @@
         \__unravel_scan_keyword:nF { gGoOtToO }
           {
             \__unravel_scan_keyword:nF { tThHrReEaAdD }
-              { \msg_error:nn { unravel } { action-type-missing } }
+              { \__unravel_error:nnnnn { action-type-missing } { } { } { } { } }
           }
       }
     \__unravel_scan_keyword:nT { fFiIlLeE }
@@ -3634,7 +3709,7 @@
           {
             \__unravel_scan_keyword:nTF { nNuUmM }
               { \__unravel_scan_int: }
-              { \msg_error:nn { unravel } { identifier-type-missing } }
+              { \__unravel_error:nnnnn { identifier-type-missing } { } { } { } { } }
           }
       }
     \__unravel_scan_keyword:nTF { nNeEwWwWiInNdDoOwW }
@@ -3687,9 +3762,9 @@
 \cs_new_protected_nopar:Npn \__unravel_scan_immediate_operands_bad:
   {
     \__unravel_back_input:
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpop:N \l__unravel_head_tl
     \__unravel_print_action:x { \tl_to_str:N \l__unravel_head_tl ignored }
-    \seq_gput_right:Nn \g__unravel_prev_input_seq { }
+    \__unravel_prev_input_gpush:
   }
 
 \cs_new_protected_nopar:Npn \__unravel_scan_pdfdest_operands:
@@ -3699,7 +3774,7 @@
       {
         \__unravel_scan_keyword:nTF { nNaAmMeE }
           { \__unravel_scan_pdf_ext_toks: }
-          { \msg_error:nn { unravel } { identifier-type-missing } }
+          { \__unravel_error:nnnnn { identifier-type-missing } { } { } { } { } }
       }
     \__unravel_scan_keyword:nTF { xXyYzZ }
       {
@@ -3728,10 +3803,8 @@
                                 \__unravel_scan_keyword:nF
                                   { fFiItT }
                                   {
-                                    \msg_error:nn { unravel }
-                                      {
-                                        destination-type-missing
-                                      }
+                                    \__unravel_error:nnnnn { destination-type-missing }
+                                      { } { } { } { }
                                   }
                               }
                           }
@@ -3744,7 +3817,7 @@
   }
 \cs_set_protected_nopar:Npn \__unravel_tmp:w
   {
-    \seq_gput_right:Nn \g__unravel_prev_input_seq { }
+    \__unravel_prev_input_gpush:
     \__unravel_prefixed_command:
   }
 \int_step_inline:nnnn
@@ -3763,10 +3836,11 @@
         \int_compare:nNnF \l__unravel_head_cmd_int
           > { \__unravel_tex_use:n { max_non_prefixed_command } }
           {
-            \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_tmpa_tl
-            \msg_error:nnxx { unravel  } { erroneous-prefixes }
+            \__unravel_prev_input_gpop:N \l__unravel_tmpa_tl
+            \__unravel_error:nxxxx { erroneous-prefixes }
               { \tl_to_str:N \l__unravel_tmpa_tl }
               { \tl_to_str:N l__unravel_head_tl }
+              { } { }
             \__unravel_back_input:
             \__unravel_omit_after_assignment:w
           }
@@ -3776,7 +3850,7 @@
     \cs_if_exist_use:cF
       { __unravel_prefixed_ \int_use:N \l__unravel_head_cmd_int : }
       {
-        \msg_error:nnx { unravel } { internal } { prefixed }
+        \__unravel_error:nnnnn { internal } { prefixed } { } { } { }
         \__unravel_omit_after_assignment:w
       }
     \__unravel_after_assignment:
@@ -3795,14 +3869,14 @@
   }
 \cs_new_protected:Npn \__unravel_assign_token:n #1
   {
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpop:N \l__unravel_head_tl
     #1
     \tl_use:N \l__unravel_head_tl \scan_stop:
     \__unravel_print_assigned_token:
   }
 \cs_new_protected_nopar:Npn \__unravel_assign_register:
   {
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpop:N \l__unravel_head_tl
     \tl_use:N \l__unravel_head_tl \scan_stop:
     \__unravel_print_assigned_register:
   }
@@ -3810,10 +3884,10 @@
   {
     \tl_if_empty:nF {#1}
       {
-        \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpush:N \l__unravel_head_tl
         \__unravel_print_action:x { \tl_to_str:N \l__unravel_head_tl }
         #1
-        \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpop:N \l__unravel_head_tl
       }
     \__unravel_prev_input:V \l__unravel_head_tl
     \tl_set_eq:NN \l__unravel_defined_tl \l__unravel_head_tl
@@ -3825,10 +3899,10 @@
   {
     \int_compare:nNnT \l__unravel_head_char_int = \c_zero
       { % \toks
-        \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpush:N \l__unravel_head_tl
         \__unravel_print_action:
         \__unravel_scan_int:
-        \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpop:N \l__unravel_head_tl
       }
     \__unravel_assign_toks:
   }
@@ -3921,7 +3995,7 @@
   }
 \__unravel_prefixed_new:nn { set_font }                            % 87
   {
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_tmpa_tl
+    \__unravel_prev_input_gpop:N \l__unravel_tmpa_tl
     \tl_put_left:NV \l__unravel_head_tl \l__unravel_tmpa_tl
     \tl_use:N \l__unravel_head_tl \scan_stop:
     \gtl_gput_right:NV \g__unravel_output_gtl \l__unravel_head_tl
@@ -3948,11 +4022,11 @@
   }
 \__unravel_prefixed_new:nn { let }                                 % 94
   {
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpush:N \l__unravel_head_tl
     \token_if_eq_meaning:NNTF \l__unravel_head_token \tex_let:D
       { % |let|
         \__unravel_scan_r_token:
-        \seq_get_right:NN \g__unravel_prev_input_seq \l__unravel_tmpa_tl
+        \__unravel_prev_input_get:N \l__unravel_tmpa_tl
         \__unravel_print_action:x { \tl_to_str:N \l__unravel_tmpa_tl }
         \__unravel_get_next:
         \bool_while_do:nn
@@ -3965,7 +4039,7 @@
       }
       { % |futurelet|
         \__unravel_scan_r_token:
-        \seq_get_right:NN \g__unravel_prev_input_seq \l__unravel_tmpa_tl
+        \__unravel_prev_input_get:N \l__unravel_tmpa_tl
         \__unravel_print_action:x { \tl_to_str:N \l__unravel_tmpa_tl }
         \__unravel_get_next:
         \gtl_set_eq:NN \l__unravel_tmpb_gtl \l__unravel_head_gtl
@@ -3974,10 +4048,10 @@
         \gtl_set_eq:NN \l__unravel_head_gtl \l__unravel_tmpb_gtl
         \__unravel_back_input:
       }
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_tmpa_tl
+    \__unravel_prev_input_gpop:N \l__unravel_tmpa_tl
     \tl_put_right:Nn \l__unravel_tmpa_tl { = ~ \l__unravel_head_token }
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
-    \use:x
+    \__unravel_prev_input_gpop:N \l__unravel_head_tl
+    \__unravel_exp_args:Nx \use:n
       {
         \exp_not:V \l__unravel_head_tl
         \tex_let:D \tl_tail:N \l__unravel_tmpa_tl
@@ -4004,12 +4078,12 @@
     \__unravel_scan_int:
     \__unravel_scan_to:
     \__unravel_scan_r_token:
-    \seq_get_right:NN \g__unravel_prev_input_seq \l__unravel_tmpa_tl
+    \__unravel_prev_input_get:N \l__unravel_tmpa_tl
     \__unravel_read_to_cs_safe:fTF
       { \__unravel_tl_first_int:N \l__unravel_tmpa_tl }
       { \__unravel_assign_token:n { } }
       {
-        \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpop:N \l__unravel_head_tl
         \__unravel_tex_fatal_error:nV { cannot-read } \l__unravel_head_tl
       }
   }
@@ -4036,10 +4110,10 @@
 \cs_generate_variant:Nn \__unravel_read_to_cs_safe:nTF { f }
 \__unravel_prefixed_new:nn { def }                                 % 97
   {
-    \seq_get_right:NN \g__unravel_prev_input_seq \l__unravel_tmpa_tl
+    \__unravel_prev_input_get:N \l__unravel_tmpa_tl
     \tl_set:NV \l__unravel_defining_tl \l__unravel_tmpa_tl
     \tl_put_right:NV \l__unravel_defining_tl \l__unravel_head_tl
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpush:N \l__unravel_head_tl
     \int_compare:nNnTF \l__unravel_head_char_int < \c_two
       { % def/gdef
         \__unravel_scan_r_token:
@@ -4051,7 +4125,7 @@
         \tl_put_right:NV \l__unravel_defining_tl \l__unravel_defined_tl
         \__unravel_scan_toks:NN \c_true_bool \c_true_bool
       }
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpop:N \l__unravel_head_tl
     \__unravel_prev_input:V \l__unravel_head_tl
     \__unravel_assign_token:n
       { \tl_set_eq:NN \l__unravel_head_tl \l__unravel_defining_tl }
@@ -4064,8 +4138,8 @@
     \bool_if:NTF \g__unravel_set_box_allowed_bool
       { \__unravel_do_box:N \c_false_bool }
       {
-        \msg_error:nn { unravel } { improper-setbox }
-        \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_tmpa_tl
+        \__unravel_error:nnnnn { improper-setbox } { } { } { } { }
+        \__unravel_prev_input_gpop:N \l__unravel_tmpa_tl
         \__unravel_omit_after_assignment:w
       }
   }
@@ -4077,7 +4151,7 @@
   }
 \__unravel_prefixed_new:nn { set_interaction }                     % 100
   {
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_tmpa_tl
+    \__unravel_prev_input_gpop:N \l__unravel_tmpa_tl
     \tl_put_left:NV \l__unravel_head_tl \l__unravel_tmpa_tl
     \tl_use:N \l__unravel_head_tl \scan_stop:
     \__unravel_print_action:x { \tl_to_str:N \l__unravel_head_tl }
@@ -4148,8 +4222,8 @@
   }
 \cs_new_protected_nopar:Npn \__unravel_do_operation_fail:w
   {
-    \msg_error:nn { unravel } { after-advance }
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_tmpa_tl
+    \__unravel_error:nnnnn { after-advance } { } { } { } { }
+    \__unravel_prev_input_gpop:N \l__unravel_tmpa_tl
     \__unravel_omit_after_assignment:w
   }
 \cs_new_protected:Npn \__unravel_do_register:N #1
@@ -4161,10 +4235,10 @@
   {
     \int_compare:nNnTF { \tl_tail:n {#2} } = \c_zero
       {
-        \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpush:N \l__unravel_head_tl
         \__unravel_print_action:
         \__unravel_scan_int:
-        \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpop:N \l__unravel_head_tl
         \__unravel_prev_input_silent:V \l__unravel_head_tl
       }
       {
@@ -4194,7 +4268,7 @@
             { 3 } { \__unravel_scan_normal_glue:  } % glue
             { 4 } { \__unravel_scan_mu_glue:      } % muglue
           }
-          { \msg_error:nnx { unravel } { internal } { do-reg=#2 } }
+          { \__unravel_error:nxxxx { internal } { do-reg=#2 } { } { } { } }
       }
       { \__unravel_scan_int: }
     \__unravel_assign_register:
@@ -4208,7 +4282,7 @@
       > { \__unravel_tex_use:n { max_non_prefixed_command } }
       {
         \bool_gset_false:N \g__unravel_set_box_allowed_bool
-        \seq_gput_right:Nn \g__unravel_prev_input_seq { }
+        \__unravel_prev_input_gpush:
         \__unravel_prefixed_command:
         \bool_gset_true:N \g__unravel_set_box_allowed_bool
         \__unravel_do_assignments:
@@ -4227,13 +4301,13 @@
     \__unravel_get_next:
     \gtl_concat:NNN \l__unravel_head_gtl
       \l__unravel_tmpb_gtl \l__unravel_head_gtl
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_gtl
+    \__unravel_prev_input_gpush_gtl:N \l__unravel_head_gtl
     \__unravel_print_action:x { \gtl_to_str:N \l__unravel_head_gtl }
     \__unravel_get_next:
     \__unravel_token_if_expandable:NTF \l__unravel_head_token
       { \__unravel_expand: }
       { \__unravel_back_input: }
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_gtl
+    \__unravel_prev_input_gpop:N \l__unravel_head_gtl
     \__unravel_set_action_text:x
       { back_input: ~ \gtl_to_str:N \l__unravel_head_gtl }
     \gtl_pop_left:N \l__unravel_head_gtl
@@ -4258,7 +4332,7 @@
   }
 \cs_new_protected_nopar:Npn \__unravel_unless_bad:
   {
-    \msg_error:nn { unravel } { bad-unless }
+    \__unravel_error:nnnnn { bad-unless } { } { } { } { }
     \__unravel_back_input:
   }
 \__unravel_new_tex_expandable:nn { no_expand }                     % 105
@@ -4283,7 +4357,7 @@
       }
   }
 \cs_new_protected_nopar:Npn \__unravel_pdfprimitive:
-  { \msg_error:nnx { unravel } { not-implemented } { pdfprimitive } }
+  { \__unravel_not_implemented:n { pdfprimitive } }
 \__unravel_new_tex_expandable:nn { input }                       % 106
   {
     \int_case:nnF \l__unravel_head_char_int
@@ -4298,23 +4372,25 @@
   }
 \cs_new_protected_nopar:Npn \__unravel_endinput:
   {
-    \msg_warning:nn { unravel } { endinput-ignored }
+    \group_begin:
+      \msg_warning:nn { unravel } { endinput-ignored }
+    \group_end:
     \__unravel_print_action:
   }
 \cs_new_protected_nopar:Npn \__unravel_scantokens:
   {
-    \seq_gput_right:Nn \g__unravel_prev_input_seq { }
+    \__unravel_prev_input_gpush:
     \__unravel_scan_toks:NN \c_false_bool \c_false_bool
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_tmpa_tl
+    \__unravel_prev_input_gpop:N \l__unravel_tmpa_tl
     \tl_set_rescan:Nno \l__unravel_head_tl { } \l__unravel_tmpa_tl
     \__unravel_back_input:V \l__unravel_head_tl
     \__unravel_print_action:x { \tl_to_str:N \l__unravel_tmpa_tl }
   }
 \cs_new_protected_nopar:Npn \__unravel_input:
   {
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpush:N \l__unravel_head_tl
     \__unravel_scan_file_name:
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpop:N \l__unravel_head_tl
     \tl_set:Nx \l__unravel_tmpa_tl { \tl_tail:N \l__unravel_head_tl }
     \__unravel_file_get:nN \l__unravel_tmpa_tl \l__unravel_tmpa_tl
     \__unravel_back_input:V \l__unravel_tmpa_tl
@@ -4322,22 +4398,22 @@
   }
 \__unravel_new_tex_expandable:nn { cs_name }                       % 109
   {
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpush:N \l__unravel_head_tl
     \__unravel_print_action:
     \__unravel_csname_loop:
-    \__unravel_prev_input:V \l__unravel_head_tl
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_silent:V \l__unravel_head_tl
+    \__unravel_prev_input_gpop:N \l__unravel_head_tl
     \__unravel_back_input_tl_o:
   }
 \cs_new_protected_nopar:Npn \__unravel_csname_loop:
   {
     \__unravel_get_x_next:
-    \token_if_cs:NTF \l__unravel_head_token
+    \__unravel_gtl_if_head_is_definable:NTF \l__unravel_head_gtl
       {
         \cs_if_eq:NNF \l__unravel_head_token \tex_endcsname:D
           {
-            \msg_error:nn { unravel } { missing-endcsname }
             \__unravel_back_input:
+            \__unravel_tex_error:nV { missing-endcsname } \l__unravel_head_tl
             \tl_set:Nn \l__unravel_head_tl { \tex_endcsname:D }
           }
       }
@@ -4349,7 +4425,7 @@
   }
 \__unravel_new_tex_expandable:nn { convert }                       % 110
   {
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpush:N \l__unravel_head_tl
     \__unravel_print_action:
     \int_case:nn \l__unravel_head_char_int
       {
@@ -4385,7 +4461,7 @@
         { 30 } \__unravel_scan_int:
         { 31 } \__unravel_scan_pdfximagebbox:
       }
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpop:N \l__unravel_head_tl
     \__unravel_back_input_tl_o:
   }
 \cs_new_protected_nopar:Npn \__unravel_scan_pdfstrcmp:
@@ -4427,18 +4503,18 @@
   }
 \cs_new_protected_nopar:Npn \__unravel_get_the:
   {
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpush:N \l__unravel_head_tl
     \__unravel_print_action:
     \int_if_odd:nTF \l__unravel_head_char_int
       { % \unexpanded, \detokenize
         \__unravel_scan_toks:NN \c_false_bool \c_false_bool
-        \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpop:N \l__unravel_head_tl
         \__unravel_set_action_text:x { \tl_to_str:N \l__unravel_head_tl }
       }
       { % \the
         \__unravel_get_x_next:
         \__unravel_scan_something_internal:n { 5 }
-        \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpop:N \l__unravel_head_tl
         \__unravel_set_action_text:x
           {
             \tl_head:N \l__unravel_head_tl
@@ -4452,7 +4528,7 @@
   { \__unravel_back_input_tl_o: }
 \__unravel_new_tex_expandable:nn { end_template }                  % 117
   {
-    \msg_error:nn { unravel } { not-implemented } { end-template }
+    \__unravel_not_implemented:n { end-template } { } { } { }
     \__unravel_back_input_tl_o:
   }
 \cs_new_protected_nopar:Npn \__unravel_pass_text:
@@ -4505,7 +4581,7 @@
   }
 \cs_new_protected_nopar:Npn \__unravel_pass_text_empty:
   {
-    \msg_error:nn { unravel } { runaway-if }
+    \__unravel_error:nnnnn { runaway-if } { } { } { } { }
     \__unravel_exit:w
   }
 \cs_new_protected:Npn \__unravel_cond_push:
@@ -4554,7 +4630,7 @@
         { 21 } { \__unravel_test_pdfprimitive:n {#1} } % ^^A todo and \unless
       }
       {
-        \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpush:N \l__unravel_head_tl
         \__unravel_print_action:
         \int_case:nn \l__unravel_head_char_int
           {
@@ -4585,7 +4661,7 @@
             { 23 } % ifpdfabsdim
               { \__unravel_test_two_vals:N \__unravel_scan_normal_dimen: }
           }
-        \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+        \__unravel_prev_input_gpop:N \l__unravel_head_tl
         \__unravel_set_action_text:x { \tl_to_str:N \l__unravel_head_tl }
         \l__unravel_head_tl \scan_stop:
           \exp_after:wN \__unravel_cond_true:n
@@ -4613,7 +4689,7 @@
       {
         \token_if_eq_meaning:NNT \l__unravel_head_token \or:
           {
-            \msg_error:nn { unravel } { extra-or }
+            \__unravel_error:nnnnn { extra-or } { } { } { } { }
             \__unravel_cond_false_loop:n {#1}
           }
       }
@@ -4635,7 +4711,7 @@
     \__unravel_get_x_non_blank:
     \tl_if_in:nVF { < = > } \l__unravel_head_tl
       {
-        \msg_error:nn { unravel } { missing-equals }
+        \__unravel_error:nnnnn { missing-equals } { } { } { } { }
         \__unravel_back_input:
         \tl_set:Nn \l__unravel_head_tl { = }
       }
@@ -4664,12 +4740,12 @@
   }
 \cs_new_protected:Npn \__unravel_test_ifx:n #1
   {
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpush:N \l__unravel_head_tl
     \__unravel_print_action:
     \__unravel_get_next:
     \gtl_set_eq:NN \l__unravel_tmpb_gtl \l__unravel_head_gtl
     \__unravel_get_next:
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_tmpa_tl
+    \__unravel_prev_input_gpop:N \l__unravel_tmpa_tl
     \__unravel_set_action_text:x
       {
         Compare:~ \tl_to_str:N \l__unravel_tmpa_tl
@@ -4687,15 +4763,15 @@
   { \gtl_head_do:NN \l__unravel_head_gtl \l__unravel_tmpa_tl }
 \cs_new_protected:Npn \__unravel_test_case:n #1
   {
-    \seq_gput_right:NV \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpush:N \l__unravel_head_tl
     \__unravel_print_action:
     \bool_if:NT \g__unravel_internal_debug_bool { \iow_term:n { {\ifcase level~#1} } }
     \__unravel_scan_int:
-    \seq_get_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_get:N \l__unravel_head_tl
     \tl_set:Nx \l__unravel_head_tl { \tl_tail:N \l__unravel_head_tl }
     % ^^A does text_case_aux use prev_input_seq?
     \exp_args:No \__unravel_test_case_aux:nn { \l__unravel_head_tl } {#1}
-    \seq_gpop_right:NN \g__unravel_prev_input_seq \l__unravel_head_tl
+    \__unravel_prev_input_gpop:N \l__unravel_head_tl
     \__unravel_print_action:x { \tl_to_str:N \l__unravel_head_tl }
   }
 \cs_new_protected:Npn \__unravel_test_case_aux:nn #1#2
@@ -4751,10 +4827,10 @@
         \int_compare:nNnTF \g__unravel_if_limit_int = \c_zero
           {
             \int_compare:nNnTF \g__unravel_if_depth_int = \c_zero
-              { \msg_error:nn { unravel } { extra-fi-or-else } }
+              { \__unravel_error:nnnnn { extra-fi-or-else } { } { } { } { } }
               { \__unravel_insert_relax: }
           }
-          { \msg_error:nn { unravel } { extra-fi-or-else } }
+          { \__unravel_error:nnnnn { extra-fi-or-else } { } { } { } { } }
       }
       {
         \__unravel_set_action_text:
@@ -4781,8 +4857,25 @@
         \__unravel_fi_or_else_loop:
       }
   }
-\cs_new_eq:NN \__unravel_print:n \iow_term:n
-\cs_generate_variant:Nn \__unravel_print:n { x }
+\tl_new:N \l__unravel_print_tl
+\sys_if_engine_luatex:TF
+  {
+    \cs_new_protected:Npx \__unravel_print_normalize_null:
+      {
+        \tl_replace_all:Nnn \exp_not:N \l__unravel_print_tl
+          { \char_generate:nn { 0 } { 12 } }
+          { \tl_to_str:n { ^ ^ @ } }
+      }
+  }
+  { \cs_new_protected:Npn \__unravel_print_normalize_null: { } }
+\cs_new_protected:Npn \__unravel_print:n #1
+  {
+    \tl_set:Nn \l__unravel_print_tl {#1}
+    \__unravel_print_normalize_null:
+    \__unravel_exp_args:Nx \iow_term:n { \l__unravel_print_tl }
+  }
+\cs_new_protected:Npn \__unravel_print:x
+  { \__unravel_exp_args:Nx \__unravel_print:n }
 \cs_new_protected:Npn \__unravel_print_message:nn #1 #2
   { \iow_wrap:nnnN { #1 #2 } { #1 } { } \__unravel_print:n }
 \cs_new_protected:Npn \__unravel_set_action_text:x #1
@@ -4808,19 +4901,18 @@
       \tl_use:N \g__unravel_before_print_state_tl
       \int_compare:nNnT \g__unravel_noise_int > \c_zero
         {
-          \exp_args:Nx \__unravel_print_state_output:n
-            { \gtl_to_str:N \g__unravel_output_gtl }
-          \seq_set_map:NNn \l__unravel_tmpa_seq \g__unravel_prev_input_seq
-            { \__unravel_to_str:n {##1} }
-          \seq_remove_all:Nn \l__unravel_tmpa_seq { }
-          \exp_args:Nx \__unravel_print_state_prev:n
-            { \seq_use:Nn \l__unravel_tmpa_seq { \\ } }
-          \exp_args:Nx \__unravel_print_state_input:n
-            { \__unravel_input_to_str: }
+          \__unravel_print_state_output:
+          \__unravel_print_state_prev:
+          \__unravel_print_state_input:
         }
     \group_end:
     \__unravel_prompt:
   }
+\cs_new_protected_nopar:Npn \__unravel_print_state_output:
+  {
+    \__unravel_exp_args:Nx \__unravel_print_state_output:n
+      { \gtl_to_str:N \g__unravel_output_gtl }
+  }
 \cs_new_protected:Npn \__unravel_print_state_output:n #1
   {
     \tl_if_empty:nF {#1}
@@ -4829,8 +4921,25 @@
           { \__unravel_str_truncate_left:nn {#1} { \g__unravel_max_output_int } }
       }
   }
-\cs_new_protected:Npn \__unravel_print_state_prev:n #1
-  { \__unravel_print_message:nn { || ~ } {#1} }
+\cs_new_protected_nopar:Npn \__unravel_print_state_prev:
+  {
+    \seq_set_map:NNn \l__unravel_tmpa_seq \g__unravel_prev_input_seq
+      { \__unravel_to_str:n {##1} }
+    \seq_remove_all:Nn \l__unravel_tmpa_seq { }
+    \seq_if_empty:NTF \l__unravel_tmpa_seq
+      { \__unravel_print_message:nn { || ~ } { } }
+      {
+        \seq_map_inline:Nn \l__unravel_tmpa_seq
+          {
+            \__unravel_print_message:nn { || ~ } {##1}
+          }
+      }
+  }
+\cs_new_protected_nopar:Npn \__unravel_print_state_input:
+  {
+    \__unravel_exp_args:Nx \__unravel_print_state_input:n
+      { \__unravel_input_to_str: }
+  }
 \cs_new_protected:Npn \__unravel_print_state_input:n #1
   {
     \__unravel_print_message:nn { |> ~ }
@@ -4892,7 +5001,7 @@
 \cs_new_protected_nopar:Npn \__unravel_print_assigned_register:
   {
     \__unravel_after_assignment: % ^^A todo: simplify
-    \exp_args:Nx \__unravel_print_action:x
+    \__unravel_exp_args:Nx \__unravel_print_action:x
       {
         \exp_not:n
           {
@@ -4944,8 +5053,8 @@
     \int_compare:nNnT { \etex_interactionmode:D } = { 3 }
       {
         \bool_if:NTF \g__unravel_explicit_prompt_bool
-          { \ior_get_str:Nc \c__unravel_prompt_ior }
-          { \ior_get_str:Nc \c__unravel_noprompt_ior }
+          { \ior_str_get:Nc \c__unravel_prompt_ior }
+          { \ior_str_get:Nc \c__unravel_noprompt_ior }
               { Your~input }
         \exp_args:Nv \__unravel_prompt_treat:n { Your~input }
       }
@@ -4954,7 +5063,7 @@
   {
     \tl_if_empty:nF {#1}
       {
-        \exp_args:Nx \str_case:nnF { \tl_head:n {#1} }
+        \__unravel_exp_args:Nx \str_case:nnF { \tl_head:n {#1} }
           {
             { m } { \__unravel_print_meaning: \__unravel_prompt_aux: }
             { q }
@@ -4974,8 +5083,11 @@
               { \int_gset:Nn \g__unravel_noise_int } }
             { C }
               {
-                \tl_gset_rescan:Nnx \g__unravel_tmpc_tl
-                  { \ExplSyntaxOn } { \tl_tail:n {#1} }
+                \__unravel_exp_args:Nx \use:n
+                  {
+                    \tl_gset_rescan:Nnn \exp_not:N \g__unravel_tmpc_tl
+                      { \exp_not:N \ExplSyntaxOn } { \tl_tail:n {#1} }
+                  }
                 \tl_gput_left:Nn \g__unravel_tmpc_tl
                   { \tl_gclear:N \g__unravel_tmpc_tl }
                 \group_insert_after:N \g__unravel_tmpc_tl
@@ -5037,8 +5149,8 @@
     \tl_gset:Nf \g__unravel_before_print_state_tl
       {
         \exp_args:NNf \exp_stop_f: \int_compare:nNnTF
-          { \int_eval:n { \seq_count:N \g__unravel_prev_input_seq - #2 } }
-          #1 { \seq_count:N \g__unravel_prev_input_seq }
+          { \int_eval:n { \__unravel_prev_input_count: - #2 } }
+          #1 { \__unravel_prev_input_count: }
           {
             \int_gset:Nn \g__unravel_nonstop_int
               { \int_max:nn { \g__unravel_nonstop_int } { 2 } }
@@ -5067,6 +5179,20 @@
   }
 \cs_new:Npn \__unravel_prompt_all_aux:N #1
   { \exp_not:n { \int_gset:Nn #1 } { \int_use:N #1 } }
+\cs_new_protected:Npn \__unravel_not_implemented:n #1
+  { \__unravel_error:nnnnn { not-implemented } {#1} { } { } { } }
+\cs_new_protected:Npn \__unravel_error:nnnnn #1#2#3#4#5
+  {
+    \group_begin:
+    \msg_error:nnnnnn { unravel } {#1} {#2} {#3} {#4} {#5}
+    \group_end:
+  }
+\cs_new_protected:Npn \__unravel_error:nxxxx #1#2#3#4#5
+  {
+    \group_begin:
+    \msg_error:nnxxxx { unravel } {#1} {#2} {#3} {#4} {#5}
+    \group_end:
+  }
 \cs_new_protected:Npn \__unravel_tex_msg_new:nnn #1#2#3
   {
     \cs_new_nopar:cpn { __unravel_tex_msg_error_#1: } {#2}
@@ -5074,18 +5200,21 @@
   }
 \cs_new_protected:Npn \__unravel_tex_error:nn #1#2
   {
+    \group_begin:
     \msg_error:nnxxx { unravel } { tex-error }
       { \tl_to_str:n {#2} }
       { \use:c { __unravel_tex_msg_error_#1: } }
       { \use:c { __unravel_tex_msg_help_#1: } }
+    \group_end:
   }
 \cs_generate_variant:Nn \__unravel_tex_error:nn { nV }
 \cs_new_protected:Npn \__unravel_tex_fatal_error:nn #1#2
   {
-    \msg_error:nnxxx { unravel } { tex-fatal }
+    \__unravel_error:nxxxx { tex-fatal }
       { \tl_to_str:n {#2} }
       { \use:c { __unravel_tex_msg_error_#1: } }
       { \use:c { __unravel_tex_msg_help_#1: } }
+      { }
   }
 \cs_generate_variant:Nn \__unravel_tex_fatal_error:nn { nV }
 \keys_define:nn { unravel/defaults }
@@ -5189,15 +5318,13 @@
   }
 \cs_new_protected_nopar:Npn \__unravel_final_bad:
   {
-    \msg_error:nnx { unravel } { internal }
-      { the-last-unravel-finished-badly }
+    \__unravel_error:nnnnn { internal }
+      { the-last-unravel-finished-badly } { } { } { }
   }
 \msg_new:nnn { unravel } { unknown-primitive }
   { Internal~error:~the~primitive~'#1'~is~not~known. }
 \msg_new:nnn { unravel } { extra-fi-or-else }
   { Extra~fi,~or,~or~else. }
-\msg_new:nnn { unravel } { missing-lbrace }
-  { Missing~left~brace~inserted. }
 \msg_new:nnn { unravel } { missing-dollar }
   { Missing~dollar~inserted. }
 \msg_new:nnn { unravel } { unknown-expandable }
@@ -5206,8 +5333,6 @@
   { Missing~font~identifier.~\iow_char:N\\nullfont~inserted. }
 \msg_new:nnn { unravel } { missing-rparen }
   { Missing~right~parenthesis~inserted~for~expression. }
-\msg_new:nnn { unravel } { missing-mudim }
-  { Missing~mu~unit. }
 \msg_new:nnn { unravel } { missing-cs }
   { Missing~control~sequence.~\iow_char:N\\inaccessible~inserted. }
 \msg_new:nnn { unravel } { missing-box }
@@ -5243,8 +5368,6 @@
   }
 \msg_new:nnn { unravel } { bad-unless }
   { \iow_char:N\\unless~not~followed~by~conditional. }
-\msg_new:nnn { unravel } { missing-endcsname }
-  { Missing~\iow_char:N\\endcsname~inserted. }
 \msg_new:nnn { unravel } { runaway-if }
   { Runaway~\iow_char:N\\if... }
 \msg_new:nnn { unravel } { runaway-macro-parameter }
@@ -5281,11 +5404,11 @@
 \__unravel_tex_msg_new:nnn { incompatible-mag }
   {
     Incompatible~magnification~
-    ( \int_to_arabic:n { \__unravel_mag: } ); \\
-    \ the~previous~value~will~be~retained
+    ( \int_to_arabic:n { \__unravel_mag: } );~
+    the~previous~value~will~be~retained
   }
   {
-    I~can~handle~only~one~magnification~ratio~per~job.~So~I've\\
+    I~can~handle~only~one~magnification~ratio~per~job.~So~I've~
     reverted~to~the~magnification~you~used~earlier~on~this~run.
   }
 \__unravel_tex_msg_new:nnn { illegal-mag }
@@ -5294,9 +5417,55 @@
     ( \int_to_arabic:n { \__unravel_mag: } )
   }
   { The~magnification~ratio~must~be~between~1~and~32768. }
+\__unravel_tex_msg_new:nnn { missing-number }
+  { Missing~number,~treated~as~zero }
+  {
+    A~number~should~have~been~here;~I~inserted~`0'.~
+    If~you~can't~figure~out~why~I~needed~to~see~a~number,~
+    look~up~`weird~error'~in~the~index~to~The~TeXbook.
+  }
+\__unravel_tex_msg_new:nnn { the-cannot }
+  { You~can't~use~`\tl_to_str:N\l__unravel_head_tl'~after~\iow_char:N\\the }
+  { I'm~forgetting~what~you~said~and~using~zero~instead. }
 \__unravel_tex_msg_new:nnn { incompatible-units }
   { Incompatible~glue~units }
   { I'm~going~to~assume~that~1mu=1pt~when~they're~mixed. }
+\__unravel_tex_msg_new:nnn { missing-mu }
+  { Illegal~unit~of~measure~(mu~inserted) }
+  {
+    The~unit~of~measurement~in~math~glue~must~be~mu.~
+    To~recover~gracefully~from~this~error,~it's~best~to~
+    delete~the~erroneous~units;~e.g.,~type~`2'~to~delete~
+    two~letters.~(See~Chapter~27~of~The~TeXbook.)
+  }
+\__unravel_tex_msg_new:nnn { missing-pt }
+  { Illegal~unit~of~measure~(pt~inserted) }
+  {
+    Dimensions~can~be~in~units~of~em,~ex,~in,~pt,~pc,~
+    cm,~mm,~dd,~cc,~nd,~nc,~bp,~or~sp;~but~yours~is~a~new~one!~
+    I'll~assume~that~you~meant~to~say~pt,~for~printer's~points.~
+    To~recover~gracefully~from~this~error,~it's~best~to~
+    delete~the~erroneous~units;~e.g.,~type~`2'~to~delete~
+    two~letters.~(See~Chapter~27~of~The~TeXbook.)
+  }
+\__unravel_tex_msg_new:nnn { missing-lbrace }
+  { Missing~\iow_char:N\{~inserted }
+  {
+    A~left~brace~was~mandatory~here,~so~I've~put~one~in.~
+    You~might~want~to~delete~and/or~insert~some~corrections~
+    so~that~I~will~find~a~matching~right~brace~soon.~
+    (If~you're~confused~by~all~this,~try~typing~`I\iow_char:N\}'~now.)
+  }
+\__unravel_tex_msg_new:nnn { extra-endcsname }
+  { Extra~\token_to_str:c{endcsname} }
+  { I'm~ignoring~this,~since~I~wasn't~doing~a~\token_to_str:c{csname}. }
+\__unravel_tex_msg_new:nnn { missing-endcsname }
+  { Missing~\token_to_str:c{endcsname}~inserted }
+  {
+    The~control~sequence~marked~<to~be~read~again>~should~
+    not~appear~between~\token_to_str:c{csname}~and~
+    \token_to_str:c{endcsname}.
+  }
 \__unravel_tex_msg_new:nnn { cannot-read }
   { ***~(cannot~\iow_char:N\\read~from~terminal~in~nonstop~modes) }
   { }
-- 
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