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+% \iffalse meta-comment
+%
+%% File: l3tl.dtx
+%
+% Copyright (C) 1990-2019 The LaTeX3 Project
+%
+% It may be distributed and/or modified under the conditions of the
+% LaTeX Project Public License (LPPL), either version 1.3c of this
+% license or (at your option) any later version. The latest version
+% of this license is in the file
+%
+% https://www.latex-project.org/lppl.txt
+%
+% This file is part of the "l3kernel bundle" (The Work in LPPL)
+% and all files in that bundle must be distributed together.
+%
+% -----------------------------------------------------------------------
+%
+% The development version of the bundle can be found at
+%
+% https://github.com/latex3/latex3
+%
+% for those people who are interested.
+%
+%<*driver>
+\documentclass[full,kernel]{l3doc}
+\begin{document}
+ \DocInput{\jobname.dtx}
+\end{document}
+%</driver>
+% \fi
+%
+% \title{^^A
+% The \pkg{l3tl} package\\ Token lists^^A
+% }
+%
+% \author{^^A
+% The \LaTeX3 Project\thanks
+% {^^A
+% E-mail:
+% \href{mailto:latex-team@latex-project.org}
+% {latex-team@latex-project.org}^^A
+% }^^A
+% }
+%
+% \date{Released 2019-08-25}
+%
+% \maketitle
+%
+% \begin{documentation}
+%
+% \TeX{} works with tokens, and \LaTeX3 therefore provides a number of
+% functions to deal with lists of tokens. Token lists may be present
+% directly in the argument to a function:
+% \begin{verbatim}
+% \foo:n { a collection of \tokens }
+% \end{verbatim}
+% or may be stored in a so-called \enquote{token list variable}, which
+% have the suffix \texttt{tl}: a token list variable can also be used as
+% the argument to a function, for example
+% \begin{verbatim}
+% \foo:N \l_some_tl
+% \end{verbatim}
+% In both cases, functions are available to test and manipulate the lists
+% of tokens, and these have the module prefix \texttt{tl}.
+% In many cases, functions which can be applied to token list variables
+% are paired with similar functions for application to explicit lists
+% of tokens: the two \enquote{views} of a token list are therefore collected
+% together here.
+%
+% A token list (explicit, or stored in a variable) can be seen either
+% as a list of \enquote{items},
+% or a list of \enquote{tokens}. An item is whatever \cs{use:n} would
+% grab as its argument: a single non-space token or a brace group,
+% with optional leading explicit space characters (each item is thus
+% itself a token list). A token is either a normal \texttt{N} argument,
+% or \verb*| |, |{|, or |}| (assuming normal \TeX{} category codes).
+% Thus for example
+% \begin{verbatim}
+% { Hello } ~ world
+% \end{verbatim}
+% contains six items (\texttt{Hello}, \texttt{w}, \texttt{o}, \texttt{r},
+% \texttt{l} and \texttt{d}), but thirteen tokens (|{|, \texttt{H}, \texttt{e},
+% \texttt{l}, \texttt{l}, \texttt{o}, |}|, \verb*| |, \texttt{w}, \texttt{o},
+% \texttt{r}, \texttt{l} and \texttt{d}).
+% Functions which act on items are often faster than their analogue acting
+% directly on tokens.
+%
+% \section{Creating and initialising token list variables}
+%
+% \begin{function}{\tl_new:N, \tl_new:c}
+% \begin{syntax}
+% \cs{tl_new:N} \meta{tl~var}
+% \end{syntax}
+% Creates a new \meta{tl~var} or raises an error if the
+% name is already taken. The declaration is global. The
+% \meta{tl~var} is initially empty.
+% \end{function}
+%
+% \begin{function}{\tl_const:Nn, \tl_const:Nx, \tl_const:cn, \tl_const:cx}
+% \begin{syntax}
+% \cs{tl_const:Nn} \meta{tl~var} \Arg{token list}
+% \end{syntax}
+% Creates a new constant \meta{tl~var} or raises an error
+% if the name is already taken. The value of the
+% \meta{tl~var} is set globally to the \meta{token list}.
+% \end{function}
+%
+% \begin{function}{\tl_clear:N, \tl_clear:c, \tl_gclear:N, \tl_gclear:c}
+% \begin{syntax}
+% \cs{tl_clear:N} \meta{tl~var}
+% \end{syntax}
+% Clears all entries from the \meta{tl~var}.
+% \end{function}
+%
+% \begin{function}
+% {\tl_clear_new:N, \tl_clear_new:c, \tl_gclear_new:N, \tl_gclear_new:c}
+% \begin{syntax}
+% \cs{tl_clear_new:N} \meta{tl~var}
+% \end{syntax}
+% Ensures that the \meta{tl~var} exists globally by applying
+% \cs{tl_new:N} if necessary, then applies \cs[index=tl_clear:N]{tl_(g)clear:N} to leave
+% the \meta{tl~var} empty.
+% \end{function}
+%
+% \begin{function}
+% {
+% \tl_set_eq:NN, \tl_set_eq:cN, \tl_set_eq:Nc, \tl_set_eq:cc,
+% \tl_gset_eq:NN, \tl_gset_eq:cN, \tl_gset_eq:Nc, \tl_gset_eq:cc
+% }
+% \begin{syntax}
+% \cs{tl_set_eq:NN} \meta{tl~var_1} \meta{tl~var_2}
+% \end{syntax}
+% Sets the content of \meta{tl~var_1} equal to that of
+% \meta{tl~var_2}.
+% \end{function}
+%
+% \begin{function}[added = 2012-05-18]
+% {
+% \tl_concat:NNN, \tl_concat:ccc,
+% \tl_gconcat:NNN, \tl_gconcat:ccc
+% }
+% \begin{syntax}
+% \cs{tl_concat:NNN} \meta{tl~var_1} \meta{tl~var_2} \meta{tl~var_3}
+% \end{syntax}
+% Concatenates the content of \meta{tl~var_2} and \meta{tl~var_3}
+% together and saves the result in \meta{tl~var_1}. The \meta{tl~var_2}
+% is placed at the left side of the new token list.
+% \end{function}
+%
+% \begin{function}[EXP, pTF, added=2012-03-03]{\tl_if_exist:N, \tl_if_exist:c}
+% \begin{syntax}
+% \cs{tl_if_exist_p:N} \meta{tl~var}
+% \cs{tl_if_exist:NTF} \meta{tl~var} \Arg{true code} \Arg{false code}
+% \end{syntax}
+% Tests whether the \meta{tl~var} is currently defined. This does not
+% check that the \meta{tl~var} really is a token list variable.
+% \end{function}
+%
+% \section{Adding data to token list variables}
+%
+% \begin{function}
+% {
+% \tl_set:Nn, \tl_set:NV, \tl_set:Nv, \tl_set:No, \tl_set:Nf, \tl_set:Nx,
+% \tl_set:cn, \tl_set:cV, \tl_set:cv, \tl_set:co, \tl_set:cf, \tl_set:cx,
+% \tl_gset:Nn, \tl_gset:NV, \tl_gset:Nv,
+% \tl_gset:No, \tl_gset:Nf, \tl_gset:Nx,
+% \tl_gset:cn, \tl_gset:cV, \tl_gset:cv,
+% \tl_gset:co, \tl_gset:cf, \tl_gset:cx
+% }
+% \begin{syntax}
+% \cs{tl_set:Nn} \meta{tl~var} \Arg{tokens}
+% \end{syntax}
+% Sets \meta{tl~var} to contain \meta{tokens},
+% removing any previous content from the variable.
+% \end{function}
+%
+% \begin{function}
+% {
+% \tl_put_left:Nn, \tl_put_left:NV, \tl_put_left:No, \tl_put_left:Nx,
+% \tl_put_left:cn, \tl_put_left:cV, \tl_put_left:co, \tl_put_left:cx,
+% \tl_gput_left:Nn, \tl_gput_left:NV, \tl_gput_left:No, \tl_gput_left:Nx,
+% \tl_gput_left:cn, \tl_gput_left:cV, \tl_gput_left:co, \tl_gput_left:cx
+% }
+% \begin{syntax}
+% \cs{tl_put_left:Nn} \meta{tl~var} \Arg{tokens}
+% \end{syntax}
+% Appends \meta{tokens} to the left side of the current content of
+% \meta{tl~var}.
+% \end{function}
+%
+% \begin{function}
+% {
+% \tl_put_right:Nn, \tl_put_right:NV, \tl_put_right:No, \tl_put_right:Nx,
+% \tl_put_right:cn, \tl_put_right:cV, \tl_put_right:co, \tl_put_right:cx,
+% \tl_gput_right:Nn, \tl_gput_right:NV, \tl_gput_right:No,
+% \tl_gput_right:Nx,
+% \tl_gput_right:cn, \tl_gput_right:cV, \tl_gput_right:co,
+% \tl_gput_right:cx
+% }
+% \begin{syntax}
+% \cs{tl_put_right:Nn} \meta{tl~var} \Arg{tokens}
+% \end{syntax}
+% Appends \meta{tokens} to the right side of the current content of
+% \meta{tl~var}.
+% \end{function}
+%
+% \section{Modifying token list variables}
+%
+% \begin{function}[updated = 2011-08-11]
+% {
+% \tl_replace_once:Nnn, \tl_replace_once:cnn,
+% \tl_greplace_once:Nnn, \tl_greplace_once:cnn
+% }
+% \begin{syntax}
+% \cs{tl_replace_once:Nnn} \meta{tl~var} \Arg{old tokens} \Arg{new tokens}
+% \end{syntax}
+% Replaces the first (leftmost) occurrence of \meta{old tokens} in the
+% \meta{tl~var} with \meta{new tokens}. \meta{Old tokens}
+% cannot contain |{|, |}| or |#|
+% (more precisely, explicit character tokens with category code $1$
+% (begin-group) or $2$ (end-group), and tokens with category code $6$).
+% \end{function}
+%
+% \begin{function}[updated = 2011-08-11]
+% {
+% \tl_replace_all:Nnn, \tl_replace_all:cnn,
+% \tl_greplace_all:Nnn, \tl_greplace_all:cnn
+% }
+% \begin{syntax}
+% \cs{tl_replace_all:Nnn} \meta{tl~var} \Arg{old tokens} \Arg{new tokens}
+% \end{syntax}
+% Replaces all occurrences of \meta{old tokens} in the
+% \meta{tl~var} with \meta{new tokens}. \meta{Old tokens}
+% cannot contain |{|, |}| or |#|
+% (more precisely, explicit character tokens with category code $1$
+% (begin-group) or $2$ (end-group), and tokens with category code $6$).
+% As this function
+% operates from left to right, the pattern \meta{old tokens}
+% may remain after the replacement (see \cs{tl_remove_all:Nn}
+% for an example).
+% \end{function}
+%
+% \begin{function}[updated = 2011-08-11]
+% {
+% \tl_remove_once:Nn, \tl_remove_once:cn,
+% \tl_gremove_once:Nn, \tl_gremove_once:cn
+% }
+% \begin{syntax}
+% \cs{tl_remove_once:Nn} \meta{tl~var} \Arg{tokens}
+% \end{syntax}
+% Removes the first (leftmost) occurrence of \meta{tokens} from the
+% \meta{tl~var}. \meta{Tokens} cannot contain |{|, |}| or |#|
+% (more precisely, explicit character tokens with category code $1$
+% (begin-group) or $2$ (end-group), and tokens with category code $6$).
+% \end{function}
+%
+% \begin{function}[updated = 2011-08-11]
+% {
+% \tl_remove_all:Nn, \tl_remove_all:cn,
+% \tl_gremove_all:Nn, \tl_gremove_all:cn
+% }
+% \begin{syntax}
+% \cs{tl_remove_all:Nn} \meta{tl~var} \Arg{tokens}
+% \end{syntax}
+% Removes all occurrences of \meta{tokens} from the
+% \meta{tl~var}. \meta{Tokens} cannot contain |{|, |}| or |#|
+% (more precisely, explicit character tokens with category code $1$
+% (begin-group) or $2$ (end-group), and tokens with category code $6$).
+% As this function
+% operates from left to right, the pattern \meta{tokens}
+% may remain after the removal, for instance,
+% \begin{quote}
+% \cs{tl_set:Nn} \cs{l_tmpa_tl} |{abbccd}|
+% \cs{tl_remove_all:Nn} \cs{l_tmpa_tl} |{bc}|
+% \end{quote}
+% results in \cs{l_tmpa_tl} containing \texttt{abcd}.
+% \end{function}
+%
+% \section{Reassigning token list category codes}
+%
+% These functions allow the rescanning of tokens: re-apply \TeX{}'s
+% tokenization process to apply category codes different from those
+% in force when the tokens were absorbed. Whilst this functionality is
+% supported, it is often preferable to find alternative approaches
+% to achieving outcomes rather than rescanning tokens (for example
+% construction of token lists token-by-token with intervening category
+% code changes or using \cs{char_generate:nn}).
+%
+% \begin{function}[updated = 2015-08-11]
+% {
+% \tl_set_rescan:Nnn, \tl_set_rescan:Nno, \tl_set_rescan:Nnx,
+% \tl_set_rescan:cnn, \tl_set_rescan:cno, \tl_set_rescan:cnx,
+% \tl_gset_rescan:Nnn, \tl_gset_rescan:Nno, \tl_gset_rescan:Nnx,
+% \tl_gset_rescan:cnn, \tl_gset_rescan:cno, \tl_gset_rescan:cnx
+% }
+% \begin{syntax}
+% \cs{tl_set_rescan:Nnn} \meta{tl~var} \Arg{setup} \Arg{tokens}
+% \end{syntax}
+% Sets \meta{tl~var} to contain \meta{tokens}, applying the category
+% code r\'{e}gime specified in the \meta{setup} before carrying out
+% the assignment. (Category codes applied to tokens not explicitly covered
+% by the \meta{setup} are those in force at the point of use of
+% \cs{tl_set_rescan:Nnn}.)
+% This allows the \meta{tl~var} to contain material
+% with category codes other than those that apply when \meta{tokens}
+% are absorbed. The \meta{setup} is run within a group and may
+% contain any valid input, although only changes in category codes
+% are relevant. See also \cs{tl_rescan:nn}.
+% \begin{texnote}
+% The \meta{tokens} are first turned into a string (using
+% \cs{tl_to_str:n}). If the string contains one or more characters
+% with character code \tn{newlinechar} (set equal to
+% \tn{endlinechar} unless that is equal to $32$, before the user
+% \meta{setup}), then it is split into lines at these characters,
+% then read as if reading multiple lines from a file, ignoring
+% spaces (catcode $10$) at the beginning and spaces and tabs
+% (character code $32$ or $9$) at the end of every line.
+% Otherwise, spaces (and tabs) are retained at both ends of the
+% single-line string, as if it appeared in the middle of a line
+% read from a file.
+% \end{texnote}
+% \end{function}
+%
+% \begin{function}[updated = 2015-08-11]{\tl_rescan:nn}
+% \begin{syntax}
+% \cs{tl_rescan:nn} \Arg{setup} \Arg{tokens}
+% \end{syntax}
+% Rescans \meta{tokens} applying the category code r\'{e}gime
+% specified in the \meta{setup}, and leaves the resulting tokens in
+% the input stream. (Category codes applied to tokens not explicitly covered
+% by the \meta{setup} are those in force at the point of use of
+% \cs{tl_rescan:nn}.)
+% The \meta{setup} is run within a group and may
+% contain any valid input, although only changes in category codes
+% are relevant. See also \cs{tl_set_rescan:Nnn}, which is more
+% robust than using \cs{tl_set:Nn} in the \meta{tokens} argument of
+% \cs{tl_rescan:nn}.
+% \begin{texnote}
+% The \meta{tokens} are first turned into a string (using
+% \cs{tl_to_str:n}). If the string contains one or more characters
+% with character code \tn{newlinechar} (set equal to
+% \tn{endlinechar} unless that is equal to $32$, before the user
+% \meta{setup}), then it is split into lines at these characters,
+% then read as if reading multiple lines from a file, ignoring
+% spaces (catcode $10$) at the beginning and spaces and tabs
+% (character code $32$ or $9$) at the end of every line.
+% Otherwise, spaces (and tabs) are retained at both ends of the
+% single-line string, as if it appeared in the middle of a line
+% read from a file.
+% \end{texnote}
+% \end{function}
+%
+% \section{Token list conditionals}
+%
+% \begin{function}[EXP,pTF]{\tl_if_blank:n, \tl_if_blank:V, \tl_if_blank:o}
+% \begin{syntax}
+% \cs{tl_if_blank_p:n} \Arg{token list}
+% \cs{tl_if_blank:nTF} \Arg{token list} \Arg{true code} \Arg{false code}
+% \end{syntax}
+% Tests if the \meta{token list} consists only of blank spaces
+% (\emph{i.e.}~contains no item). The test is \texttt{true} if
+% \meta{token list} is zero or more explicit space characters
+% (explicit tokens with character code~$32$ and category code~$10$),
+% and is \texttt{false} otherwise.
+% \end{function}
+%
+% \begin{function}[EXP,pTF]{\tl_if_empty:N, \tl_if_empty:c}
+% \begin{syntax}
+% \cs{tl_if_empty_p:N} \meta{tl~var}
+% \cs{tl_if_empty:NTF} \meta{tl~var} \Arg{true code} \Arg{false code}
+% \end{syntax}
+% Tests if the \meta{token list variable} is entirely empty
+% (\emph{i.e.}~contains no tokens at all).
+% \end{function}
+%
+% \begin{function}[added = 2012-05-24, updated = 2012-06-05, EXP,pTF]
+% {\tl_if_empty:n, \tl_if_empty:V, \tl_if_empty:o}
+% \begin{syntax}
+% \cs{tl_if_empty_p:n} \Arg{token list}
+% \cs{tl_if_empty:nTF} \Arg{token list} \Arg{true code} \Arg{false code}
+% \end{syntax}
+% Tests if the \meta{token list} is entirely empty
+% (\emph{i.e.}~contains no tokens at all).
+% \end{function}
+%
+% \begin{function}[EXP,pTF]
+% {\tl_if_eq:NN, \tl_if_eq:Nc, \tl_if_eq:cN, \tl_if_eq:cc}
+% \begin{syntax}
+% \cs{tl_if_eq_p:NN} \meta{tl~var_1} \meta{tl~var_2}
+% \cs{tl_if_eq:NNTF} \meta{tl~var_1} \meta{tl~var_2} \Arg{true code} \Arg{false code}
+% \end{syntax}
+% Compares the content of two \meta{token list variables} and
+% is logically \texttt{true} if the two contain the same list of
+% tokens (\emph{i.e.}~identical in both the list of characters they
+% contain and the category codes of those characters). Thus for example
+% \begin{verbatim}
+% \tl_set:Nn \l_tmpa_tl { abc }
+% \tl_set:Nx \l_tmpb_tl { \tl_to_str:n { abc } }
+% \tl_if_eq:NNTF \l_tmpa_tl \l_tmpb_tl { true } { false }
+% \end{verbatim}
+% yields \texttt{false}.
+% \end{function}
+%
+% \begin{function}[TF]{\tl_if_eq:nn}
+% \begin{syntax}
+% \cs{tl_if_eq:nnTF} \Arg{token list_1} \Arg{token list_2} \Arg{true code} \Arg{false code}
+% \end{syntax}
+% Tests if \meta{token list_1} and \meta{token list_2} contain the
+% same list of tokens, both in respect of character codes and category
+% codes.
+% \end{function}
+%
+% \begin{function}[TF]{\tl_if_in:Nn, \tl_if_in:cn}
+% \begin{syntax}
+% \cs{tl_if_in:NnTF} \meta{tl~var} \Arg{token list} \Arg{true code} \Arg{false code}
+% \end{syntax}
+% Tests if the \meta{token list} is found in the content of the
+% \meta{tl~var}. The \meta{token list} cannot contain
+% the tokens |{|, |}| or |#|
+% (more precisely, explicit character tokens with category code $1$
+% (begin-group) or $2$ (end-group), and tokens with category code $6$).
+% \end{function}
+%
+% \begin{function}[TF]
+% {\tl_if_in:nn, \tl_if_in:Vn, \tl_if_in:on, \tl_if_in:no}
+% \begin{syntax}
+% \cs{tl_if_in:nnTF} \Arg{token list_1} \Arg{token list_2} \Arg{true code} \Arg{false code}
+% \end{syntax}
+% Tests if \meta{token list_2} is found inside \meta{token list_1}.
+% The \meta{token list_2} cannot contain the tokens |{|, |}| or |#|
+% (more precisely, explicit character tokens with category code $1$
+% (begin-group) or $2$ (end-group), and tokens with category code $6$).
+% \end{function}
+%
+% \begin{function}[added = 2017-11-14, EXP,pTF]{\tl_if_novalue:n}
+% \begin{syntax}
+% \cs{tl_if_novalue_p:n} \Arg{token list}
+% \cs{tl_if_novalue:nTF} \Arg{token list} \Arg{true code} \Arg{false code}
+% \end{syntax}
+% Tests if the \meta{token list} is exactly equal to the special
+% \cs{c_novalue_tl} marker. This function is intended to allow construction
+% of flexible document interface structures in which missing optional
+% arguments are detected.
+% \end{function}
+%
+% \begin{function}[updated = 2011-08-13, EXP,pTF]
+% {\tl_if_single:N, \tl_if_single:c}
+% \begin{syntax}
+% \cs{tl_if_single_p:N} \meta{tl~var}
+% \cs{tl_if_single:NTF} \meta{tl~var} \Arg{true code} \Arg{false code}
+% \end{syntax}
+% Tests if the content of the \meta{tl~var} consists of a single item,
+% \emph{i.e.}~is a single normal token (neither an explicit space
+% character nor a begin-group character) or a single brace group,
+% surrounded by optional spaces on both sides. In other words, such a
+% token list has token count $1$ according to \cs{tl_count:N}.
+% \end{function}
+%
+% \begin{function}[updated = 2011-08-13, EXP,pTF]{\tl_if_single:n}
+% \begin{syntax}
+% \cs{tl_if_single_p:n} \Arg{token list}
+% \cs{tl_if_single:nTF} \Arg{token list} \Arg{true code} \Arg{false code}
+% \end{syntax}
+% Tests if the \meta{token list} has exactly one item, \emph{i.e.}~is
+% a single normal token (neither an explicit space character nor a
+% begin-group character) or a single brace group, surrounded by
+% optional spaces on both sides. In other words, such a token list has
+% token count $1$ according to \cs{tl_count:n}.
+% \end{function}
+%
+% \begin{function}[EXP,pTF]{\tl_if_single_token:n}
+% \begin{syntax}
+% \cs{tl_if_single_token_p:n} \Arg{token list}
+% \cs{tl_if_single_token:nTF} \Arg{token list} \Arg{true code} \Arg{false code}
+% \end{syntax}
+% Tests if the token list consists of exactly one token, \emph{i.e.}~is
+% either a single space character or a single \enquote{normal} token.
+% Token groups (|{|\ldots|}|) are not single tokens.
+% \end{function}
+%
+% \begin{function}[added = 2013-07-24, EXP, noTF]{\tl_case:Nn, \tl_case:cn}
+% \begin{syntax}
+% \cs{tl_case:NnTF} \meta{test token list variable} \\
+% ~~"{" \\
+% ~~~~\meta{token list variable case_1} \Arg{code case_1} \\
+% ~~~~\meta{token list variable case_2} \Arg{code case_2} \\
+% ~~~~\ldots \\
+% ~~~~\meta{token list variable case_n} \Arg{code case_n} \\
+% ~~"}" \\
+% ~~\Arg{true code}
+% ~~\Arg{false code}
+% \end{syntax}
+% This function compares the \meta{test token list variable} in turn
+% with each of the \meta{token list variable cases}. If the two
+% are equal (as described for \cs{tl_if_eq:NNTF})
+% then the associated \meta{code} is left in the input
+% stream and other cases are discarded. If any of the
+% cases are matched, the \meta{true code} is also inserted into the
+% input stream (after the code for the appropriate case), while if none
+% match then the \meta{false code} is inserted. The function
+% \cs{tl_case:Nn}, which does nothing if there is no match, is also
+% available.
+% \end{function}
+%
+% \section{Mapping to token lists}
+%
+% All mappings are done at the current group level, \emph{i.e.}~any
+% local assignments made by the \meta{function} or \meta{code} discussed
+% below remain in effect after the loop.
+%
+% \begin{function}[updated = 2012-06-29, rEXP]
+% {\tl_map_function:NN, \tl_map_function:cN}
+% \begin{syntax}
+% \cs{tl_map_function:NN} \meta{tl~var} \meta{function}
+% \end{syntax}
+% Applies \meta{function} to every \meta{item} in the \meta{tl~var}.
+% The \meta{function} receives one argument for each iteration.
+% This may be a number of tokens if the \meta{item} was stored within
+% braces. Hence the \meta{function} should anticipate receiving
+% \texttt{n}-type arguments. See also \cs{tl_map_function:nN}.
+% \end{function}
+%
+% \begin{function}[updated = 2012-06-29, rEXP]{\tl_map_function:nN}
+% \begin{syntax}
+% \cs{tl_map_function:nN} \Arg{token list} \meta{function}
+% \end{syntax}
+% Applies \meta{function} to every \meta{item} in the \meta{token list},
+% The \meta{function} receives one argument for each iteration.
+% This may be a number of tokens if the \meta{item} was stored within
+% braces. Hence the \meta{function} should anticipate receiving
+% \texttt{n}-type arguments. See also \cs{tl_map_function:NN}.
+% \end{function}
+%
+% \begin{function}[updated = 2012-06-29]
+% {\tl_map_inline:Nn, \tl_map_inline:cn}
+% \begin{syntax}
+% \cs{tl_map_inline:Nn} \meta{tl~var} \Arg{inline function}
+% \end{syntax}
+% Applies the \meta{inline function} to every \meta{item} stored within the
+% \meta{tl~var}. The \meta{inline function} should consist of code which
+% receives the \meta{item} as |#1|. See also \cs{tl_map_function:NN}.
+% \end{function}
+%
+% \begin{function}[updated = 2012-06-29]{\tl_map_inline:nn}
+% \begin{syntax}
+% \cs{tl_map_inline:nn} \Arg{token list} \Arg{inline function}
+% \end{syntax}
+% Applies the \meta{inline function} to every \meta{item} stored within the
+% \meta{token list}. The \meta{inline function} should consist of code which
+% receives the \meta{item} as |#1|. See also \cs{tl_map_function:nN}.
+% \end{function}
+%
+% \begin{function}[updated = 2012-06-29]
+% {\tl_map_variable:NNn, \tl_map_variable:cNn}
+% \begin{syntax}
+% \cs{tl_map_variable:NNn} \meta{tl~var} \meta{variable} \Arg{code}
+% \end{syntax}
+% Stores each \meta{item} of the \meta{tl~var} in turn in the (token
+% list) \meta{variable} and applies the \meta{code}. The \meta{code}
+% will usually make use of the \meta{variable}, but this is not
+% enforced. The assignments to the \meta{variable} are local. Its
+% value after the loop is the last \meta{item} in the \meta{tl~var},
+% or its original value if the \meta{tl~var} is blank. See also
+% \cs{tl_map_inline:Nn}.
+% \end{function}
+%
+% \begin{function}[updated = 2012-06-29]{\tl_map_variable:nNn}
+% \begin{syntax}
+% \cs{tl_map_variable:nNn} \Arg{token list} \meta{variable} \Arg{code}
+% \end{syntax}
+% Stores each \meta{item} of the \meta{token list} in turn in the
+% (token list) \meta{variable} and applies the \meta{code}. The
+% \meta{code} will usually make use of the \meta{variable}, but this
+% is not enforced. The assignments to the \meta{variable} are local.
+% Its value after the loop is the last \meta{item} in the
+% \meta{tl~var}, or its original value if the \meta{tl~var} is blank.
+% See also \cs{tl_map_inline:nn}.
+% \end{function}
+%
+% \begin{function}[updated = 2012-06-29, rEXP]{\tl_map_break:}
+% \begin{syntax}
+% \cs{tl_map_break:}
+% \end{syntax}
+% Used to terminate a \cs[no-index]{tl_map_\ldots} function before all
+% entries in the \meta{token list variable} have been processed. This
+% normally takes place within a conditional statement, for example
+% \begin{verbatim}
+% \tl_map_inline:Nn \l_my_tl
+% {
+% \str_if_eq:nnT { #1 } { bingo } { \tl_map_break: }
+% % Do something useful
+% }
+% \end{verbatim}
+% See also \cs{tl_map_break:n}.
+% Use outside of a \cs[no-index]{tl_map_\ldots} scenario leads to low
+% level \TeX{} errors.
+% \begin{texnote}
+% When the mapping is broken, additional tokens may be inserted
+% before the \meta{tokens} are
+% inserted into the input stream.
+% This depends on the design of the mapping function.
+% \end{texnote}
+% \end{function}
+%
+% \begin{function}[updated = 2012-06-29, rEXP]{\tl_map_break:n}
+% \begin{syntax}
+% \cs{tl_map_break:n} \Arg{code}
+% \end{syntax}
+% Used to terminate a \cs[no-index]{tl_map_\ldots} function before all
+% entries in the \meta{token list variable} have been processed, inserting
+% the \meta{code} after the mapping has ended. This
+% normally takes place within a conditional statement, for example
+% \begin{verbatim}
+% \tl_map_inline:Nn \l_my_tl
+% {
+% \str_if_eq:nnT { #1 } { bingo }
+% { \tl_map_break:n { <code> } }
+% % Do something useful
+% }
+% \end{verbatim}
+% Use outside of a \cs[no-index]{tl_map_\ldots} scenario leads to low
+% level \TeX{} errors.
+% \begin{texnote}
+% When the mapping is broken, additional tokens may be inserted
+% before the \meta{code} is
+% inserted into the input stream.
+% This depends on the design of the mapping function.
+% \end{texnote}
+% \end{function}
+%
+% \section{Using token lists}
+%
+% \begin{function}[EXP]{\tl_to_str:n, \tl_to_str:V}
+% \begin{syntax}
+% \cs{tl_to_str:n} \Arg{token list}
+% \end{syntax}
+% Converts the \meta{token list} to a \meta{string}, leaving the
+% resulting character tokens in the input stream. A \meta{string}
+% is a series of tokens with category code $12$ (other) with the exception
+% of spaces, which retain category code $10$ (space).
+% This function requires only a single expansion.
+% Its argument \emph{must} be braced.
+% \begin{texnote}
+% This is the \eTeX{} primitive \tn{detokenize}.
+% Converting a \meta{token list} to a \meta{string} yields a
+% concatenation of the string representations of every token in the
+% \meta{token list}.
+% The string representation of a control sequence is
+% \begin{itemize}
+% \item an escape character, whose character code is given by the
+% internal parameter \tn{escapechar}, absent if the
+% \tn{escapechar} is negative or greater than the largest
+% character code;
+% \item the control sequence name, as defined by \cs{cs_to_str:N};
+% \item a space, unless the control sequence name is a single
+% character whose category at the time of expansion of
+% \cs{tl_to_str:n} is not \enquote{letter}.
+% \end{itemize}
+% The string representation of an explicit character token is that
+% character, doubled in the case of (explicit) macro parameter
+% characters (normally |#|).
+% In particular, the string representation of a token list may
+% depend on the category codes in effect when it is evaluated, and
+% the value of the \tn{escapechar}: for instance |\tl_to_str:n {\a}|
+% normally produces the three character \enquote{backslash},
+% \enquote{lower-case a}, \enquote{space}, but it may also produce a
+% single \enquote{lower-case a} if the escape character is negative
+% and \texttt{a} is currently not a letter.
+% \end{texnote}
+% \end{function}
+%
+% \begin{function}[EXP]{\tl_to_str:N, \tl_to_str:c}
+% \begin{syntax}
+% \cs{tl_to_str:N} \meta{tl~var}
+% \end{syntax}
+% Converts the content of the \meta{tl~var} into a series of characters
+% with category code $12$ (other) with the exception of spaces, which
+% retain category code $10$ (space). This \meta{string} is then left
+% in the input stream. For low-level details, see the notes given for
+% \cs{tl_to_str:n}.
+% \end{function}
+%
+% \begin{function}[EXP]{\tl_use:N, \tl_use:c}
+% \begin{syntax}
+% \cs{tl_use:N} \meta{tl~var}
+% \end{syntax}
+% Recovers the content of a \meta{tl~var} and places it
+% directly in the input stream. An error is raised if the variable
+% does not exist or if it is invalid. Note that it is possible to use
+% a \meta{tl~var} directly without an accessor function.
+% \end{function}
+%
+% \section{Working with the content of token lists}
+%
+% \begin{function}[added = 2012-05-13, EXP]
+% {\tl_count:n, \tl_count:V, \tl_count:o}
+% \begin{syntax}
+% \cs{tl_count:n} \Arg{tokens}
+% \end{syntax}
+% Counts the number of \meta{items} in \meta{tokens} and leaves this
+% information in the input stream. Unbraced tokens count as one
+% element as do each token group (|{|\ldots|}|). This process
+% ignores any unprotected spaces within \meta{tokens}. See also
+% \cs{tl_count:N}. This function requires three expansions,
+% giving an \meta{integer denotation}.
+% \end{function}
+%
+% \begin{function}[added = 2012-05-13, EXP]{\tl_count:N, \tl_count:c}
+% \begin{syntax}
+% \cs{tl_count:N} \meta{tl~var}
+% \end{syntax}
+% Counts the number of token groups in the \meta{tl~var}
+% and leaves this information in the input stream. Unbraced tokens
+% count as one element as do each token group (|{|\ldots|}|). This
+% process ignores any unprotected spaces within the \meta{tl~var}.
+% See also \cs{tl_count:n}. This function requires three expansions,
+% giving an \meta{integer denotation}.
+% \end{function}
+%
+% \begin{function}[EXP, added = 2019-02-25]{\tl_count_tokens:n}
+% \begin{syntax}
+% \cs{tl_count_tokens:n} \Arg{tokens}
+% \end{syntax}
+% Counts the number of \TeX{} tokens in the \meta{tokens} and leaves
+% this information in the input stream. Every token, including spaces and
+% braces, contributes one to the total; thus for instance, the token count of
+% |a~{bc}| is $6$.
+% \end{function}
+%
+% \begin{function}[updated = 2012-01-08, EXP]
+% {\tl_reverse:n, \tl_reverse:V, \tl_reverse:o}
+% \begin{syntax}
+% \cs{tl_reverse:n} \Arg{token list}
+% \end{syntax}
+% Reverses the order of the \meta{items} in the \meta{token list},
+% so that \meta{item_1}\meta{item_2}\meta{item_3} \ldots \meta{item_n}
+% becomes \meta{item_n}\ldots \meta{item_3}\meta{item_2}\meta{item_1}.
+% This process preserves unprotected space within the
+% \meta{token list}. Tokens are not reversed within braced token
+% groups, which keep their outer set of braces.
+% In situations where performance is important,
+% consider \cs{tl_reverse_items:n}.
+% See also \cs{tl_reverse:N}.
+% \begin{texnote}
+% The result is returned within \tn{unexpanded}, which means that the token
+% list does not expand further when appearing in an \texttt{x}-type
+% argument expansion.
+% \end{texnote}
+% \end{function}
+%
+% \begin{function}[updated = 2012-01-08]
+% {\tl_reverse:N, \tl_reverse:c, \tl_greverse:N, \tl_greverse:c}
+% \begin{syntax}
+% \cs{tl_reverse:N} \meta{tl~var}
+% \end{syntax}
+% Reverses the order of the \meta{items} stored in \meta{tl~var}, so
+% that \meta{item_1}\meta{item_2}\meta{item_3} \ldots \meta{item_n}
+% becomes \meta{item_n}\ldots \meta{item_3}\meta{item_2}\meta{item_1}.
+% This process preserves unprotected spaces within the
+% \meta{token list variable}. Braced token groups are copied without
+% reversing the order of tokens, but keep the outer set of braces.
+% See also \cs{tl_reverse:n}, and, for improved performance,
+% \cs{tl_reverse_items:n}.
+% \end{function}
+%
+% \begin{function}[added = 2012-01-08, EXP]{\tl_reverse_items:n}
+% \begin{syntax}
+% \cs{tl_reverse_items:n} \Arg{token list}
+% \end{syntax}
+% Reverses the order of the \meta{items} stored in \meta{tl~var},
+% so that \Arg{item_1}\Arg{item_2}\Arg{item_3} \ldots \Arg{item_n}
+% becomes \Arg{item_n} \ldots{} \Arg{item_3}\Arg{item_2}\Arg{item_1}.
+% This process removes any unprotected space within the
+% \meta{token list}. Braced token groups are copied without
+% reversing the order of tokens, and keep the outer set of braces.
+% Items which are initially not braced are copied with braces in
+% the result. In cases where preserving spaces is important,
+% consider the slower function \cs{tl_reverse:n}.
+% \begin{texnote}
+% The result is returned within \tn{unexpanded}, which means that the token
+% list does not expand further when appearing in an \texttt{x}-type
+% argument expansion.
+% \end{texnote}
+% \end{function}
+%
+% \begin{function}[added = 2011-07-09, updated = 2012-06-25, EXP]
+% {\tl_trim_spaces:n, \tl_trim_spaces:o}
+% \begin{syntax}
+% \cs{tl_trim_spaces:n} \Arg{token list}
+% \end{syntax}
+% Removes any leading and trailing explicit space characters
+% (explicit tokens with character code~$32$ and category code~$10$)
+% from the \meta{token list} and leaves the result in the input
+% stream.
+% \begin{texnote}
+% The result is returned within \tn{unexpanded}, which means that the token
+% list does not expand further when appearing in an \texttt{x}-type
+% argument expansion.
+% \end{texnote}
+% \end{function}
+%
+% \begin{function}[added = 2018-04-12, EXP]
+% {\tl_trim_spaces_apply:nN, \tl_trim_spaces_apply:oN}
+% \begin{syntax}
+% \cs{tl_trim_spaces_apply:nN} \Arg{token list} \meta{function}
+% \end{syntax}
+% Removes any leading and trailing explicit space characters (explicit
+% tokens with character code~$32$ and category code~$10$) from the
+% \meta{token list} and passes the result to the \meta{function} as an
+% \texttt{n}-type argument.
+% \end{function}
+%
+% \begin{function}[added = 2011-07-09]
+% {
+% \tl_trim_spaces:N, \tl_trim_spaces:c,
+% \tl_gtrim_spaces:N, \tl_gtrim_spaces:c
+% }
+% \begin{syntax}
+% \cs{tl_trim_spaces:N} \meta{tl~var}
+% \end{syntax}
+% Removes any leading and trailing explicit space characters
+% (explicit tokens with character code~$32$ and category code~$10$)
+% from the content of the \meta{tl~var}. Note that this therefore
+% \emph{resets} the content of the variable.
+% \end{function}
+%
+% \begin{function}[added = 2017-02-06]
+% {\tl_sort:Nn, \tl_sort:cn, \tl_gsort:Nn, \tl_gsort:cn}
+% \begin{syntax}
+% \cs{tl_sort:Nn} \meta{tl var} \Arg{comparison code}
+% \end{syntax}
+% Sorts the items in the \meta{tl var} according to the
+% \meta{comparison code}, and assigns the result to
+% \meta{tl var}. The details of sorting comparison are
+% described in Section~\ref{sec:l3sort:mech}.
+% \end{function}
+%
+% \begin{function}[added = 2017-02-06, EXP]{\tl_sort:nN}
+% \begin{syntax}
+% \cs{tl_sort:nN} \Arg{token list} \meta{conditional}
+% \end{syntax}
+% Sorts the items in the \meta{token list}, using the
+% \meta{conditional} to compare items, and leaves the result in the
+% input stream. The \meta{conditional} should have signature |:nnTF|,
+% and return \texttt{true} if the two items being compared should be
+% left in the same order, and \texttt{false} if the items should be
+% swapped. The details of sorting comparison are
+% described in Section~\ref{sec:l3sort:mech}.
+% \begin{texnote}
+% The result is returned within \cs{exp_not:n}, which means that the
+% token list does not expand further when appearing in an
+% \texttt{x}-type or \texttt{e}-type argument expansion.
+% \end{texnote}
+% \end{function}
+%
+% \section{The first token from a token list}
+%
+% Functions which deal with either only the very first item (balanced
+% text or single normal token) in a token list, or the remaining tokens.
+%
+% \begin{function}[updated = 2012-09-09, EXP]
+% {\tl_head:N, \tl_head:n, \tl_head:V, \tl_head:v, \tl_head:f}
+% \begin{syntax}
+% \cs{tl_head:n} \Arg{token list}
+% \end{syntax}
+% Leaves in the input stream the first \meta{item} in the
+% \meta{token list}, discarding the rest of the \meta{token list}.
+% All leading explicit space characters
+% (explicit tokens with character code~$32$ and category code~$10$)
+% are discarded; for example
+% \begin{verbatim}
+% \tl_head:n { abc }
+% \end{verbatim}
+% and
+% \begin{verbatim}
+% \tl_head:n { ~ abc }
+% \end{verbatim}
+% both leave |a| in the input stream. If the \enquote{head} is a
+% brace group, rather than a single token, the braces are removed, and
+% so
+% \begin{verbatim}
+% \tl_head:n { ~ { ~ ab } c }
+% \end{verbatim}
+% yields \verb*| ab|.
+% A blank \meta{token list} (see \cs{tl_if_blank:nTF}) results in
+% \cs{tl_head:n} leaving nothing in the input stream.
+% \begin{texnote}
+% The result is returned within \cs{exp_not:n}, which means that the token
+% list does not expand further when appearing in an \texttt{x}-type
+% argument expansion.
+% \end{texnote}
+% \end{function}
+%
+% \begin{function}[EXP]{\tl_head:w}
+% \begin{syntax}
+% \cs{tl_head:w} \meta{token list} | { } | \cs{q_stop}
+% \end{syntax}
+% Leaves in the input stream the first \meta{item} in the
+% \meta{token list}, discarding the rest of the \meta{token list}.
+% All leading explicit space characters
+% (explicit tokens with character code~$32$ and category code~$10$)
+% are discarded.
+% A blank \meta{token list} (which consists only of space characters)
+% results in a low-level \TeX{} error, which may be avoided by the
+% inclusion of an empty group in the input (as shown), without the need
+% for an explicit test. Alternatively, \cs{tl_if_blank:nF} may be used to
+% avoid using the function with a \enquote{blank} argument.
+% This function requires only a single expansion, and thus is suitable for
+% use within an \texttt{o}-type expansion. In general, \cs{tl_head:n} should
+% be preferred if the number of expansions is not critical.
+% \end{function}
+%
+% \begin{function}[updated = 2012-09-01, EXP]
+% {\tl_tail:N, \tl_tail:n, \tl_tail:V, \tl_tail:v, \tl_tail:f}
+% \begin{syntax}
+% \cs{tl_tail:n} \Arg{token list}
+% \end{syntax}
+% Discards all leading explicit space characters
+% (explicit tokens with character code~$32$ and category code~$10$)
+% and the first \meta{item} in the \meta{token list}, and leaves the
+% remaining tokens in the input stream. Thus for example
+% \begin{verbatim}
+% \tl_tail:n { a ~ {bc} d }
+% \end{verbatim}
+% and
+% \begin{verbatim}
+% \tl_tail:n { ~ a ~ {bc} d }
+% \end{verbatim}
+% both leave \verb*| {bc}d| in the input stream. A blank
+% \meta{token list} (see \cs{tl_if_blank:nTF}) results
+% in \cs{tl_tail:n} leaving nothing in the input stream.
+% \begin{texnote}
+% The result is returned within \cs{exp_not:n}, which means that the
+% token list does not expand further when appearing in an \texttt{x}-type
+% argument expansion.
+% \end{texnote}
+% \end{function}
+%
+% \begin{function}[updated = 2012-07-09, EXP, pTF]{\tl_if_head_eq_catcode:nN}
+% \begin{syntax}
+% \cs{tl_if_head_eq_catcode_p:nN} \Arg{token list} \meta{test token}
+% \cs{tl_if_head_eq_catcode:nNTF} \Arg{token list} \meta{test token}
+% ~~\Arg{true code} \Arg{false code}
+% \end{syntax}
+% Tests if the first \meta{token} in the \meta{token list} has the
+% same category code as the \meta{test token}. In the case where the
+% \meta{token list} is empty, the test is always \texttt{false}.
+% \end{function}
+%
+% \begin{function}[updated = 2012-07-09, EXP, pTF]
+% {\tl_if_head_eq_charcode:nN, \tl_if_head_eq_charcode:fN}
+% \begin{syntax}
+% \cs{tl_if_head_eq_charcode_p:nN} \Arg{token list} \meta{test token}
+% \cs{tl_if_head_eq_charcode:nNTF} \Arg{token list} \meta{test token}
+% ~~\Arg{true code} \Arg{false code}
+% \end{syntax}
+% Tests if the first \meta{token} in the \meta{token list} has the
+% same character code as the \meta{test token}. In the case where the
+% \meta{token list} is empty, the test is always \texttt{false}.
+% \end{function}
+%
+% \begin{function}[updated = 2012-07-09, EXP, pTF]{\tl_if_head_eq_meaning:nN}
+% \begin{syntax}
+% \cs{tl_if_head_eq_meaning_p:nN} \Arg{token list} \meta{test token}
+% \cs{tl_if_head_eq_meaning:nNTF} \Arg{token list} \meta{test token}
+% ~~\Arg{true code} \Arg{false code}
+% \end{syntax}
+% Tests if the first \meta{token} in the \meta{token list} has the
+% same meaning as the \meta{test token}. In the case where
+% \meta{token list} is empty, the test is always \texttt{false}.
+% \end{function}
+%
+% \begin{function}[added = 2012-07-08, EXP, pTF]{\tl_if_head_is_group:n}
+% \begin{syntax}
+% \cs{tl_if_head_is_group_p:n} \Arg{token list}
+% \cs{tl_if_head_is_group:nTF} \Arg{token list} \Arg{true code} \Arg{false code}
+% \end{syntax}
+% Tests if the first \meta{token} in the \meta{token list}
+% is an explicit begin-group character (with category code~$1$
+% and any character code), in other words, if the \meta{token list}
+% starts with a brace group. In particular, the test is \texttt{false}
+% if the \meta{token list} starts with an implicit token such as
+% \cs{c_group_begin_token}, or if it is empty.
+% This function is useful to implement actions on token lists on
+% a token by token basis.
+% \end{function}
+%
+% \begin{function}[added = 2012-07-08, EXP, pTF]{\tl_if_head_is_N_type:n}
+% \begin{syntax}
+% \cs{tl_if_head_is_N_type_p:n} \Arg{token list}
+% \cs{tl_if_head_is_N_type:nTF} \Arg{token list} \Arg{true code} \Arg{false code}
+% \end{syntax}
+% Tests if the first \meta{token} in the \meta{token list}
+% is a normal \texttt{N}-type argument. In other words,
+% it is neither an explicit space character
+% (explicit token with character code~$32$ and category code~$10$)
+% nor an explicit begin-group character
+% (with category code~1 and any character code). An empty
+% argument yields \texttt{false}, as it does not have a \enquote{normal}
+% first token.
+% This function is useful to implement actions on token lists on
+% a token by token basis.
+% \end{function}
+%
+% \begin{function}[updated = 2012-07-08, EXP, pTF]{\tl_if_head_is_space:n}
+% \begin{syntax}
+% \cs{tl_if_head_is_space_p:n} \Arg{token list}
+% \cs{tl_if_head_is_space:nTF} \Arg{token list} \Arg{true code} \Arg{false code}
+% \end{syntax}
+% Tests if the first \meta{token} in the \meta{token list}
+% is an explicit space character
+% (explicit token with character code~$12$ and category code~$10$).
+% In particular, the test is \texttt{false} if the \meta{token list}
+% starts with an implicit token such as \cs{c_space_token}, or if it
+% is empty.
+% This function is useful to implement actions on token lists on
+% a token by token basis.
+% \end{function}
+%
+% \section{Using a single item}
+%
+% \begin{function}[added = 2014-07-17, EXP]
+% {\tl_item:nn, \tl_item:Nn, \tl_item:cn}
+% \begin{syntax}
+% \cs{tl_item:nn} \Arg{token list} \Arg{integer expression}
+% \end{syntax}
+% Indexing items in the \meta{token list} from~$1$ on the left, this
+% function evaluates the \meta{integer expression} and leaves the
+% appropriate item from the \meta{token list} in the input stream.
+% If the \meta{integer expression} is negative, indexing occurs from
+% the right of the token list, starting at $-1$ for the right-most item.
+% If the index is out of bounds, then the function expands to nothing.
+% \begin{texnote}
+% The result is returned within the \tn{unexpanded}
+% primitive (\cs{exp_not:n}), which means that the \meta{item}
+% does not expand further when appearing in an \texttt{x}-type
+% argument expansion.
+% \end{texnote}
+% \end{function}
+%
+% \begin{function}[EXP, added = 2016-12-06]
+% {\tl_rand_item:N, \tl_rand_item:c, \tl_rand_item:n}
+% \begin{syntax}
+% \cs{tl_rand_item:N} \meta{tl~var}
+% \cs{tl_rand_item:n} \Arg{token list}
+% \end{syntax}
+% Selects a pseudo-random item of the \meta{token list}. If the
+% \meta{token list} is blank, the result is empty.
+% This is not available in older versions of \XeTeX{}.
+% \begin{texnote}
+% The result is returned within the \tn{unexpanded}
+% primitive (\cs{exp_not:n}), which means that the \meta{item}
+% does not expand further when appearing in an \texttt{x}-type
+% argument expansion.
+% \end{texnote}
+% \end{function}
+%
+% \begin{function}[EXP, added = 2017-02-17, updated = 2017-07-15]
+% {\tl_range:Nnn, \tl_range:nnn}
+% \begin{syntax}
+% \cs{tl_range:Nnn} \meta{tl~var} \Arg{start index} \Arg{end index}
+% \cs{tl_range:nnn} \Arg{token list} \Arg{start index} \Arg{end index}
+% \end{syntax}
+% Leaves in the input stream the items from the \meta{start index} to the
+% \meta{end index} inclusive. Spaces and braces are preserved between
+% the items returned (but never at either end of the list).
+%
+% Here \meta{start index} and \meta{end index} should be integer denotations.
+% For describing in detail the functions' behavior, let $m$ and $n$ be the start
+% and end index respectively. If either is $0$, the result is empty. A positive
+% index means `start counting from the left end', a negative index means
+% `start counting from the right end'. Let $l$ be the count of the token list.
+%
+% The \emph{actual start point} is determined as $M=m$ if~$m>0$ and as $M=l+m+1$
+% if~$m<0$. Similarly the \emph{actual end point} is $N=n$ if~$n>0$ and $N=l+n+1$
+% if~$n<0$. If $M>N$, the result is empty. Otherwise it consists of all items from
+% position $M$ to position $N$ inclusive; for the purpose of this rule, we can
+% imagine that the token list extends at infinity on either side, with void items
+% at positions $s$ for $s\le0$ or $s>l$.
+%
+% Spaces in between items in the actual range are preserved. Spaces at either end
+% of the token list will be removed anyway (think to the token list being passed to
+% |\tl_trim_spaces:n| to begin with.
+%
+% Thus, with $l=7$ as in the examples below, all of the following are equivalent
+% and result in the whole token list
+% \begin{verbatim}
+% \tl_range:nnn { abcd~{e{}}fg } { 1 } { 7 }
+% \tl_range:nnn { abcd~{e{}}fg } { 1 } { 12 }
+% \tl_range:nnn { abcd~{e{}}fg } { -7 } { 7 }
+% \tl_range:nnn { abcd~{e{}}fg } { -12 } { 7 }
+% \end{verbatim}
+% Here are some more interesting examples. The calls
+% \begin{verbatim}
+% \iow_term:x { \tl_range:nnn { abcd{e{}}fg } { 2 } { 5 } }
+% \iow_term:x { \tl_range:nnn { abcd{e{}}fg } { 2 } { -3 } }
+% \iow_term:x { \tl_range:nnn { abcd{e{}}fg } { -6 } { 5 } }
+% \iow_term:x { \tl_range:nnn { abcd{e{}}fg } { -6 } { -3 } }
+% \end{verbatim}
+% are all equivalent and will print |bcd{e{}}| on the terminal; similarly
+% \begin{verbatim}
+% \iow_term:x { \tl_range:nnn { abcd~{e{}}fg } { 2 } { 5 } }
+% \iow_term:x { \tl_range:nnn { abcd~{e{}}fg } { 2 } { -3 } }
+% \iow_term:x { \tl_range:nnn { abcd~{e{}}fg } { -6 } { 5 } }
+% \iow_term:x { \tl_range:nnn { abcd~{e{}}fg } { -6 } { -3 } }
+% \end{verbatim}
+% are all equivalent and will print |bcd {e{}}| on the
+% terminal (note the space in the middle). To the contrary,
+% \begin{verbatim}
+% \tl_range:nnn { abcd~{e{}}f } { 2 } { 4 }
+% \end{verbatim}
+% will discard the space after `d`.
+%
+% If we want to get the items from the third to the last, the call
+% is |\tl_range:nnn { <tl> } { 3 } { -1 }|. Similarly, for discarding
+% the last item, we can do |\tl_range:nnn { <tl> } { 1 } { -2 }|.
+%
+% The behavior of \cs{tl_range:Nnn} is exactly the same, acting on the
+% contents of the tl variable.
+%
+% For improved performance, see \cs{tl_range_braced:nnn} and
+% \cs{tl_range_unbraced:nnn}.
+% \begin{texnote}
+% The result is returned within the \tn{unexpanded}
+% primitive (\cs{exp_not:n}), which means that the \meta{item}
+% does not expand further when appearing in an \texttt{x}-type
+% argument expansion.
+% \end{texnote}
+% \end{function}
+%
+% \section{Viewing token lists}
+%
+% \begin{function}[updated = 2015-08-01]{\tl_show:N, \tl_show:c}
+% \begin{syntax}
+% \cs{tl_show:N} \meta{tl~var}
+% \end{syntax}
+% Displays the content of the \meta{tl~var} on the terminal.
+% \begin{texnote}
+% This is similar to the \TeX{} primitive \tn{show}, wrapped to a
+% fixed number of characters per line.
+% \end{texnote}
+% \end{function}
+%
+% \begin{function}[updated = 2015-08-07]{\tl_show:n}
+% \begin{syntax}
+% \cs{tl_show:n} \Arg{token list}
+% \end{syntax}
+% Displays the \meta{token list} on the terminal.
+% \begin{texnote}
+% This is similar to the \eTeX{} primitive \tn{showtokens}, wrapped
+% to a fixed number of characters per line.
+% \end{texnote}
+% \end{function}
+%
+% \begin{function}[added = 2014-08-22, updated = 2015-08-01]{\tl_log:N, \tl_log:c}
+% \begin{syntax}
+% \cs{tl_log:N} \meta{tl~var}
+% \end{syntax}
+% Writes the content of the \meta{tl~var} in the log file. See also
+% \cs{tl_show:N} which displays the result in the terminal.
+% \end{function}
+%
+% \begin{function}[added = 2014-08-22, updated = 2015-08-07]{\tl_log:n}
+% \begin{syntax}
+% \cs{tl_log:n} \Arg{token list}
+% \end{syntax}
+% Writes the \meta{token list} in the log file. See also
+% \cs{tl_show:n} which displays the result in the terminal.
+% \end{function}
+%
+% \section{Constant token lists}
+%
+% \begin{variable}{\c_empty_tl}
+% Constant that is always empty.
+% \end{variable}
+%
+% \begin{variable}[added = 2017-11-14]{\c_novalue_tl}
+% A marker for the absence of an argument. This constant |tl| can safely
+% be typeset (\emph{cf.}~\cs{q_nil}), with the result being |-NoValue-|.
+% It is important to note that \cs{c_novalue_tl} is constructed such that it
+% will \emph{not} match the simple text input |-NoValue-|, \emph{i.e.}
+% that
+% \begin{verbatim}
+% \tl_if_eq:VnTF \c_novalue_tl { -NoValue- }
+% \end{verbatim}
+% is logically \texttt{false}. The \cs{c_novalue_tl} marker is intended for
+% use in creating document-level interfaces, where it serves as an indicator
+% that an (optional) argument was omitted. In particular, it is distinct
+% from a simple empty |tl|.
+% \end{variable}
+%
+% \begin{variable}{\c_space_tl}
+% An explicit space character contained in a token list (compare this with
+% \cs{c_space_token}). For use where an explicit space is required.
+% \end{variable}
+%
+% \section{Scratch token lists}
+%
+% \begin{variable}{\l_tmpa_tl, \l_tmpb_tl}
+% Scratch token lists for local assignment. These are never used by
+% the kernel code, and so are safe for use with any \LaTeX3-defined
+% function. However, they may be overwritten by other non-kernel
+% code and so should only be used for short-term storage.
+% \end{variable}
+%
+% \begin{variable}{\g_tmpa_tl, \g_tmpb_tl}
+% Scratch token lists for global assignment. These are never used by
+% the kernel code, and so are safe for use with any \LaTeX3-defined
+% function. However, they may be overwritten by other non-kernel
+% code and so should only be used for short-term storage.
+% \end{variable}
+%
+% \end{documentation}
+%
+% \begin{implementation}
+%
+% \section{\pkg{l3tl} implementation}
+%
+% \begin{macrocode}
+%<*initex|package>
+% \end{macrocode}
+%
+% \begin{macrocode}
+%<@@=tl>
+% \end{macrocode}
+%
+% A token list variable is a \TeX{} macro that holds tokens. By using the
+% \eTeX{} primitive \tn{unexpanded} inside a \TeX{} \tn{edef} it is
+% possible to store any tokens, including |#|, in this way.
+%
+% \subsection{Functions}
+%
+% \begin{macro}{\tl_new:N, \tl_new:c}
+% Creating new token list variables is a case of checking for an
+% existing definition and doing the definition.
+% \begin{macrocode}
+\cs_new_protected:Npn \tl_new:N #1
+ {
+ \__kernel_chk_if_free_cs:N #1
+ \cs_gset_eq:NN #1 \c_empty_tl
+ }
+\cs_generate_variant:Nn \tl_new:N { c }
+% \end{macrocode}
+% \end{macro}
+%
+% \begin{macro}{\tl_const:Nn, \tl_const:Nx, \tl_const:cn, \tl_const:cx}
+% Constants are also easy to generate.
+% \begin{macrocode}
+\__kernel_patch:nnNNpn { \__kernel_chk_var_scope:NN c #1 } { }
+\cs_new_protected:Npn \tl_const:Nn #1#2
+ {
+ \__kernel_chk_if_free_cs:N #1
+ \cs_gset_nopar:Npx #1 { \exp_not:n {#2} }
+ }
+\__kernel_patch:nnNNpn { \__kernel_chk_var_scope:NN c #1 } { }
+\cs_new_protected:Npn \tl_const:Nx #1#2
+ {
+ \__kernel_chk_if_free_cs:N #1
+ \cs_gset_nopar:Npx #1 {#2}
+ }
+\cs_generate_variant:Nn \tl_const:Nn { c }
+\cs_generate_variant:Nn \tl_const:Nx { c }
+% \end{macrocode}
+% \end{macro}
+%
+% \begin{macro}{\tl_clear:N, \tl_clear:c}
+% \begin{macro}{\tl_gclear:N, \tl_gclear:c}
+% Clearing a token list variable means setting it to an empty value.
+% Error checking is sorted out by the parent function.
+% \begin{macrocode}
+\cs_new_protected:Npn \tl_clear:N #1
+ { \tl_set_eq:NN #1 \c_empty_tl }
+\cs_new_protected:Npn \tl_gclear:N #1
+ { \tl_gset_eq:NN #1 \c_empty_tl }
+\cs_generate_variant:Nn \tl_clear:N { c }
+\cs_generate_variant:Nn \tl_gclear:N { c }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}{\tl_clear_new:N, \tl_clear_new:c}
+% \begin{macro}{\tl_gclear_new:N, \tl_gclear_new:c}
+% Clearing a token list variable means setting it to an empty value.
+% Error checking is sorted out by the parent function.
+% \begin{macrocode}
+\cs_new_protected:Npn \tl_clear_new:N #1
+ { \tl_if_exist:NTF #1 { \tl_clear:N #1 } { \tl_new:N #1 } }
+\cs_new_protected:Npn \tl_gclear_new:N #1
+ { \tl_if_exist:NTF #1 { \tl_gclear:N #1 } { \tl_new:N #1 } }
+\cs_generate_variant:Nn \tl_clear_new:N { c }
+\cs_generate_variant:Nn \tl_gclear_new:N { c }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}{\tl_set_eq:NN, \tl_set_eq:Nc, \tl_set_eq:cN, \tl_set_eq:cc}
+% \begin{macro}{\tl_gset_eq:NN, \tl_gset_eq:Nc, \tl_gset_eq:cN, \tl_gset_eq:cc}
+% For setting token list variables equal to each other. When checking
+% is turned on, make sure both variables exist.
+% \begin{macrocode}
+\__kernel_if_debug:TF
+ {
+ \cs_new_protected:Npn \tl_set_eq:NN #1#2
+ {
+ \__kernel_chk_var_local:N #1
+ \__kernel_chk_var_exist:N #2
+ \cs_set_eq:NN #1 #2
+ }
+ \cs_new_protected:Npn \tl_gset_eq:NN #1#2
+ {
+ \__kernel_chk_var_global:N #1
+ \__kernel_chk_var_exist:N #2
+ \cs_gset_eq:NN #1 #2
+ }
+ }
+ {
+ \cs_new_eq:NN \tl_set_eq:NN \cs_set_eq:NN
+ \cs_new_eq:NN \tl_gset_eq:NN \cs_gset_eq:NN
+ }
+\cs_generate_variant:Nn \tl_set_eq:NN { cN, Nc, cc }
+\cs_generate_variant:Nn \tl_gset_eq:NN { cN, Nc, cc }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}{\tl_concat:NNN, \tl_concat:ccc}
+% \begin{macro}{\tl_gconcat:NNN, \tl_gconcat:ccc}
+% Concatenating token lists is easy. When checking is turned on, all
+% three arguments must be checked: a token list |#2| or |#3| equal to
+% \cs{scan_stop:} would lead to problems later on.
+% \begin{macrocode}
+\__kernel_patch:nnNNpn
+ {
+ \__kernel_chk_var_exist:N #2
+ \__kernel_chk_var_exist:N #3
+ }
+ { }
+\cs_new_protected:Npn \tl_concat:NNN #1#2#3
+ { \tl_set:Nx #1 { \exp_not:o {#2} \exp_not:o {#3} } }
+\__kernel_patch:nnNNpn
+ {
+ \__kernel_chk_var_exist:N #2
+ \__kernel_chk_var_exist:N #3
+ }
+ { }
+\cs_new_protected:Npn \tl_gconcat:NNN #1#2#3
+ { \tl_gset:Nx #1 { \exp_not:o {#2} \exp_not:o {#3} } }
+\cs_generate_variant:Nn \tl_concat:NNN { ccc }
+\cs_generate_variant:Nn \tl_gconcat:NNN { ccc }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}[pTF]{\tl_if_exist:N, \tl_if_exist:c}
+% Copies of the \texttt{cs} functions defined in \pkg{l3basics}.
+% \begin{macrocode}
+\prg_new_eq_conditional:NNn \tl_if_exist:N \cs_if_exist:N { TF , T , F , p }
+\prg_new_eq_conditional:NNn \tl_if_exist:c \cs_if_exist:c { TF , T , F , p }
+% \end{macrocode}
+% \end{macro}
+%
+% \subsection{Constant token lists}
+%
+% \begin{variable}{\c_empty_tl}
+% Never full. We need to define that constant before using \cs{tl_new:N}.
+% \begin{macrocode}
+\tl_const:Nn \c_empty_tl { }
+% \end{macrocode}
+% \end{variable}
+%
+% \begin{variable}{\c_novalue_tl}
+% A special marker: as we don't have |\char_generate:nn| yet, has to be
+% created the old-fashioned way.
+% \begin{macrocode}
+\group_begin:
+\tex_lccode:D `A = `-
+\tex_lccode:D `N = `N
+\tex_lccode:D `V = `V
+\tex_lowercase:D
+ {
+ \group_end:
+ \tl_const:Nn \c_novalue_tl { ANoValue- }
+ }
+% \end{macrocode}
+% \end{variable}
+%
+% \begin{variable}{\c_space_tl}
+% A space as a token list (as opposed to as a character).
+% \begin{macrocode}
+\tl_const:Nn \c_space_tl { ~ }
+% \end{macrocode}
+% \end{variable}
+%
+% \subsection{Adding to token list variables}
+%
+% \begin{macro}
+% {
+% \tl_set:Nn, \tl_set:NV, \tl_set:Nv, \tl_set:No, \tl_set:Nf, \tl_set:Nx,
+% \tl_set:cn, \tl_set:cV, \tl_set:cv, \tl_set:co, \tl_set:cf, \tl_set:cx
+% }
+% \begin{macro}
+% {
+% \tl_gset:Nn, \tl_gset:NV, \tl_gset:Nv,
+% \tl_gset:No, \tl_gset:Nf, \tl_gset:Nx,
+% \tl_gset:cn, \tl_gset:cV, \tl_gset:cv,
+% \tl_gset:co, \tl_gset:cf, \tl_gset:cx
+% }
+% By using \cs{exp_not:n} token list variables can contain |#| tokens,
+% which makes the token list registers provided by \TeX{}
+% more or less redundant. The \cs{tl_set:No} version is done
+% \enquote{by hand} as it is used quite a lot. Each definition is
+% prefixed by a call to \cs{__kernel_patch:nnNNpn} which adds an
+% existence check to the definition.
+% \begin{macrocode}
+\__kernel_patch:nnNNpn { \__kernel_chk_var_local:N #1 } { }
+\cs_new_protected:Npn \tl_set:Nn #1#2
+ { \cs_set_nopar:Npx #1 { \exp_not:n {#2} } }
+\__kernel_patch:nnNNpn { \__kernel_chk_var_local:N #1 } { }
+\cs_new_protected:Npn \tl_set:No #1#2
+ { \cs_set_nopar:Npx #1 { \exp_not:o {#2} } }
+\__kernel_patch:nnNNpn { \__kernel_chk_var_local:N #1 } { }
+\cs_new_protected:Npn \tl_set:Nx #1#2
+ { \cs_set_nopar:Npx #1 {#2} }
+\__kernel_patch:nnNNpn { \__kernel_chk_var_global:N #1 } { }
+\cs_new_protected:Npn \tl_gset:Nn #1#2
+ { \cs_gset_nopar:Npx #1 { \exp_not:n {#2} } }
+\__kernel_patch:nnNNpn { \__kernel_chk_var_global:N #1 } { }
+\cs_new_protected:Npn \tl_gset:No #1#2
+ { \cs_gset_nopar:Npx #1 { \exp_not:o {#2} } }
+\__kernel_patch:nnNNpn { \__kernel_chk_var_global:N #1 } { }
+\cs_new_protected:Npn \tl_gset:Nx #1#2
+ { \cs_gset_nopar:Npx #1 {#2} }
+\cs_generate_variant:Nn \tl_set:Nn { NV , Nv , Nf }
+\cs_generate_variant:Nn \tl_set:Nx { c }
+\cs_generate_variant:Nn \tl_set:Nn { c, co , cV , cv , cf }
+\cs_generate_variant:Nn \tl_gset:Nn { NV , Nv , Nf }
+\cs_generate_variant:Nn \tl_gset:Nx { c }
+\cs_generate_variant:Nn \tl_gset:Nn { c, co , cV , cv , cf }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}
+% {
+% \tl_put_left:Nn, \tl_put_left:NV, \tl_put_left:No, \tl_put_left:Nx,
+% \tl_put_left:cn, \tl_put_left:cV, \tl_put_left:co, \tl_put_left:cx
+% }
+% \begin{macro}
+% {
+% \tl_gput_left:Nn, \tl_gput_left:NV, \tl_gput_left:No, \tl_gput_left:Nx,
+% \tl_gput_left:cn, \tl_gput_left:cV, \tl_gput_left:co, \tl_gput_left:cx
+% }
+% Adding to the left is done directly to gain a little performance.
+% \begin{macrocode}
+\__kernel_patch:nnNNpn { \__kernel_chk_var_local:N #1 } { }
+\cs_new_protected:Npn \tl_put_left:Nn #1#2
+ { \cs_set_nopar:Npx #1 { \exp_not:n {#2} \exp_not:o #1 } }
+\__kernel_patch:nnNNpn { \__kernel_chk_var_local:N #1 } { }
+\cs_new_protected:Npn \tl_put_left:NV #1#2
+ { \cs_set_nopar:Npx #1 { \exp_not:V #2 \exp_not:o #1 } }
+\__kernel_patch:nnNNpn { \__kernel_chk_var_local:N #1 } { }
+\cs_new_protected:Npn \tl_put_left:No #1#2
+ { \cs_set_nopar:Npx #1 { \exp_not:o {#2} \exp_not:o #1 } }
+\__kernel_patch:nnNNpn { \__kernel_chk_var_local:N #1 } { }
+\cs_new_protected:Npn \tl_put_left:Nx #1#2
+ { \cs_set_nopar:Npx #1 { #2 \exp_not:o #1 } }
+\__kernel_patch:nnNNpn { \__kernel_chk_var_global:N #1 } { }
+\cs_new_protected:Npn \tl_gput_left:Nn #1#2
+ { \cs_gset_nopar:Npx #1 { \exp_not:n {#2} \exp_not:o #1 } }
+\__kernel_patch:nnNNpn { \__kernel_chk_var_global:N #1 } { }
+\cs_new_protected:Npn \tl_gput_left:NV #1#2
+ { \cs_gset_nopar:Npx #1 { \exp_not:V #2 \exp_not:o #1 } }
+\__kernel_patch:nnNNpn { \__kernel_chk_var_global:N #1 } { }
+\cs_new_protected:Npn \tl_gput_left:No #1#2
+ { \cs_gset_nopar:Npx #1 { \exp_not:o {#2} \exp_not:o #1 } }
+\__kernel_patch:nnNNpn { \__kernel_chk_var_global:N #1 } { }
+\cs_new_protected:Npn \tl_gput_left:Nx #1#2
+ { \cs_gset_nopar:Npx #1 { #2 \exp_not:o {#1} } }
+\cs_generate_variant:Nn \tl_put_left:Nn { c }
+\cs_generate_variant:Nn \tl_put_left:NV { c }
+\cs_generate_variant:Nn \tl_put_left:No { c }
+\cs_generate_variant:Nn \tl_put_left:Nx { c }
+\cs_generate_variant:Nn \tl_gput_left:Nn { c }
+\cs_generate_variant:Nn \tl_gput_left:NV { c }
+\cs_generate_variant:Nn \tl_gput_left:No { c }
+\cs_generate_variant:Nn \tl_gput_left:Nx { c }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}
+% {
+% \tl_put_right:Nn, \tl_put_right:NV, \tl_put_right:No, \tl_put_right:Nx,
+% \tl_put_right:cn, \tl_put_right:cV, \tl_put_right:co, \tl_put_right:cx
+% }
+% \begin{macro}
+% {
+% \tl_gput_right:Nn, \tl_gput_right:NV, \tl_gput_right:No,
+% \tl_gput_right:Nx,
+% \tl_gput_right:cn, \tl_gput_right:cV, \tl_gput_right:co,
+% \tl_gput_right:cx
+% }
+% The same on the right.
+% \begin{macrocode}
+\__kernel_patch:nnNNpn { \__kernel_chk_var_local:N #1 } { }
+\cs_new_protected:Npn \tl_put_right:Nn #1#2
+ { \cs_set_nopar:Npx #1 { \exp_not:o #1 \exp_not:n {#2} } }
+\__kernel_patch:nnNNpn { \__kernel_chk_var_local:N #1 } { }
+\cs_new_protected:Npn \tl_put_right:NV #1#2
+ { \cs_set_nopar:Npx #1 { \exp_not:o #1 \exp_not:V #2 } }
+\__kernel_patch:nnNNpn { \__kernel_chk_var_local:N #1 } { }
+\cs_new_protected:Npn \tl_put_right:No #1#2
+ { \cs_set_nopar:Npx #1 { \exp_not:o #1 \exp_not:o {#2} } }
+\__kernel_patch:nnNNpn { \__kernel_chk_var_local:N #1 } { }
+\cs_new_protected:Npn \tl_put_right:Nx #1#2
+ { \cs_set_nopar:Npx #1 { \exp_not:o #1 #2 } }
+\__kernel_patch:nnNNpn { \__kernel_chk_var_global:N #1 } { }
+\cs_new_protected:Npn \tl_gput_right:Nn #1#2
+ { \cs_gset_nopar:Npx #1 { \exp_not:o #1 \exp_not:n {#2} } }
+\__kernel_patch:nnNNpn { \__kernel_chk_var_global:N #1 } { }
+\cs_new_protected:Npn \tl_gput_right:NV #1#2
+ { \cs_gset_nopar:Npx #1 { \exp_not:o #1 \exp_not:V #2 } }
+\__kernel_patch:nnNNpn { \__kernel_chk_var_global:N #1 } { }
+\cs_new_protected:Npn \tl_gput_right:No #1#2
+ { \cs_gset_nopar:Npx #1 { \exp_not:o #1 \exp_not:o {#2} } }
+\__kernel_patch:nnNNpn { \__kernel_chk_var_global:N #1 } { }
+\cs_new_protected:Npn \tl_gput_right:Nx #1#2
+ { \cs_gset_nopar:Npx #1 { \exp_not:o {#1} #2 } }
+\cs_generate_variant:Nn \tl_put_right:Nn { c }
+\cs_generate_variant:Nn \tl_put_right:NV { c }
+\cs_generate_variant:Nn \tl_put_right:No { c }
+\cs_generate_variant:Nn \tl_put_right:Nx { c }
+\cs_generate_variant:Nn \tl_gput_right:Nn { c }
+\cs_generate_variant:Nn \tl_gput_right:NV { c }
+\cs_generate_variant:Nn \tl_gput_right:No { c }
+\cs_generate_variant:Nn \tl_gput_right:Nx { c }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \subsection{Reassigning token list category codes}
+%
+% \begin{variable}{\c_@@_rescan_marker_tl}
+% The rescanning code needs a special token list containing the same
+% character (chosen here to be a colon) with two different category
+% codes: it cannot appear in the tokens being rescanned since all
+% colons have the same category code.
+% \begin{macrocode}
+\tl_const:Nx \c_@@_rescan_marker_tl { : \token_to_str:N : }
+% \end{macrocode}
+% \end{variable}
+%
+% \begin{macro}
+% {
+% \tl_set_rescan:Nnn, \tl_set_rescan:Nno, \tl_set_rescan:Nnx,
+% \tl_set_rescan:cnn, \tl_set_rescan:cno, \tl_set_rescan:cnx
+% }
+% \begin{macro}
+% {
+% \tl_gset_rescan:Nnn, \tl_gset_rescan:Nno, \tl_gset_rescan:Nnx,
+% \tl_gset_rescan:cnn, \tl_gset_rescan:cno, \tl_gset_rescan:cnx
+% }
+% \begin{macro}{\tl_rescan:nn}
+% \begin{macro}{\@@_set_rescan:NNnn, \@@_set_rescan_multi:nNN}
+% \begin{macro}[EXP]{\@@_rescan:NNw}
+% In a group, after some initial setup explained below and the user
+% setup~|#3| (followed by \cs{scan_stop:} to be safe), there is a call
+% to \cs{@@_set_rescan:nNN}. This shared auxiliary defined later
+% distinguishes single-line and multi-line ``files''. In the simplest
+% case of multi-line files, it calls (with the same arguments)
+% \cs{@@_set_rescan_multi:nNN}, whose code is included here to help
+% understand the approach. This function rescans its argument |#1|,
+% closes the group, and performs the assignment.
+%
+% One difficulty when rescanning is that \tn{scantokens} treats the
+% argument as a file, and without the correct settings a \TeX{} error
+% occurs:
+% \begin{verbatim}
+% ! File ended while scanning definition of ...
+% \end{verbatim}
+% A related minor issue is a warning due to opening a group before the
+% \tn{scantokens} and closing it inside that temporary file; we avoid
+% that by setting \tn{tracingnesting}. The standard solution to the
+% ``File ended'' error is to grab the rescanned tokens as a delimited
+% argument of an auxiliary, here \cs{@@_rescan:NNw}, that performs the
+% assignment, then let \TeX{} ``execute'' the end of file marker. As
+% usual in delimited arguments we use \cs{prg_do_nothing:} to avoid
+% stripping an outer set braces: this is removed by using
+% \texttt{o}-expanding assignments. The delimiter cannot appear
+% within the rescanned token list because it contains twice the same
+% character, with different catcodes.
+%
+% For \cs{tl_rescan:nn} we cannot simply call \cs{@@_set_rescan:NNnn}
+% \cs{prg_do_nothing:} \cs{use:n} because that would leave the
+% end-of-file marker \emph{after} the result of rescanning. If that
+% rescanned result is code that looks further in the input stream for
+% arguments, it would break.
+%
+% For multi-line files the only subtlety is that \tn{newlinechar}
+% should be equal to \tn{endlinechar} because \tn{newlinechar}
+% characters become new lines and then become \tn{endlinechar}
+% characters when writing to an abstract file and reading back. This
+% equality is ensured by setting \tn{newlinechar} equal to
+% \tn{endlinechar}. Prior to this, \tn{endlinechar} is set to $-1$ if
+% it was $32$ (in particular true after \cs{ExplSyntaxOn}) to avoid
+% unreasonable line-breaks at every space for instance in error
+% messages triggered by the user setup. Another side effect of
+% reading back from the file is that spaces (catcode $10$) are ignored
+% at the beginning of lines, and spaces and tabs (character code $32$
+% and $9$) are ignored at the end of lines.
+%
+% The two \cs{if_false:} \ldots{} \cs{fi:} are there to prevent
+% alignment tabs to cause a change of tabular cell while rescanning.
+% We put the \enquote{opening} one after \cs{group_begin:} so that if
+% one accidentally \texttt{f}-expands \cs{tl_set_rescan:Nnn} braces
+% remain balanced. This is essential in \texttt{e}-type arguments
+% when \tn{expanded} is not available.
+% \begin{macrocode}
+\cs_new_protected:Npn \tl_rescan:nn #1#2
+ {
+ \tl_set_rescan:Nnn \l_@@_internal_a_tl {#1} {#2}
+ \exp_after:wN \tl_clear:N \exp_after:wN \l_@@_internal_a_tl
+ \l_@@_internal_a_tl
+ }
+\cs_new_protected:Npn \tl_set_rescan:Nnn
+ { \@@_set_rescan:NNnn \tl_set:No }
+\cs_new_protected:Npn \tl_gset_rescan:Nnn
+ { \@@_set_rescan:NNnn \tl_gset:No }
+\cs_new_protected:Npn \@@_set_rescan:NNnn #1#2#3#4
+ {
+ \group_begin:
+ \if_false: { \fi:
+ \int_set_eq:NN \tex_tracingnesting:D \c_zero_int
+ \int_compare:nNnT \tex_endlinechar:D = { 32 }
+ { \int_set:Nn \tex_endlinechar:D { -1 } }
+ \int_set_eq:NN \tex_newlinechar:D \tex_endlinechar:D
+ #3 \scan_stop:
+ \exp_args:No \@@_set_rescan:nNN { \tl_to_str:n {#4} } #1 #2
+ \if_false: } \fi:
+ }
+\cs_new_protected:Npn \@@_set_rescan_multi:nNN #1#2#3
+ {
+ \exp_args:No \tex_everyeof:D { \c_@@_rescan_marker_tl }
+ \exp_after:wN \@@_rescan:NNw
+ \exp_after:wN #2
+ \exp_after:wN #3
+ \exp_after:wN \prg_do_nothing:
+ \tex_scantokens:D {#1}
+ }
+\exp_args:Nno \use:nn
+ { \cs_new:Npn \@@_rescan:NNw #1#2#3 } \c_@@_rescan_marker_tl
+ {
+ \group_end:
+ #1 #2 {#3}
+ }
+\cs_generate_variant:Nn \tl_set_rescan:Nnn { Nno , Nnx }
+\cs_generate_variant:Nn \tl_set_rescan:Nnn { c , cno , cnx }
+\cs_generate_variant:Nn \tl_gset_rescan:Nnn { Nno , Nnx }
+\cs_generate_variant:Nn \tl_gset_rescan:Nnn { c , cno }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}{\@@_set_rescan:nNN}
+% \begin{macro}{\@@_set_rescan_single:nnNN, \@@_set_rescan_single_aux:nnnNN}
+% \begin{macro}[rEXP]{\@@_set_rescan_single_aux:w}
+% The function \cs{@@_set_rescan:nNN} calls \cs{@@_set_rescan_multi:nNN} or
+% \cs{@@_set_rescan_single:nnNN} |{ ' }| depending on whether its argument
+% is a single-line fragment of code/data or is made of multiple lines
+% by testing for the presence of a \tn{newlinechar} character. If
+% \tn{newlinechar} is out of range, the argument is assumed to be a
+% single line.
+%
+% For a single line, no \tn{endlinechar} should be added, so it is
+% set to $-1$, and spaces should not be removed.
+% Trailing spaces and tabs are a difficult matter, as \TeX{} removes
+% these at a very low level. The only way to preserve them is to
+% rescan not the argument but the argument followed by a character
+% with a reasonable category code. Here, $11$ (letter) and $12$ (other)
+% are accepted, as these are convenient, suitable for
+% delimiting an argument, and it is very unlikely that none of the
+% ASCII characters are in one of these categories. To avoid
+% selecting one particular character to put at the end, whose
+% category code may have been modified, there is a loop through
+% characters from |'| (ASCII $39$) to |~| (ASCII $127$). The choice
+% of starting point was made because this is the start of a very long
+% range of characters whose standard category is letter or other,
+% thus minimizing the number of steps needed by the loop (most often
+% just a single one). If no valid character is found (very rare),
+% fall-back on \cs{@@_set_rescan_multi:nNN}.
+%
+% Otherwise, once a valid character is found (let us use |'| in this
+% explanation) run some code very similar to \cs{@@_set_rescan_multi:nNN}
+% but with |'| added at both ends of the input. Of course, we need to
+% define the auxiliary \cs{@@_set_rescan_single:NNww} on the fly to remove
+% the additional~|'| that is just before |::| (by which we mean
+% \cs{c_@@_rescan_marker_tl}). Note that the argument must be
+% delimited by |'| with the current catcode; this is done thanks to
+% \cs{char_generate:nn}. Yet another issue is that the rescanned
+% token list may contain a comment character, in which case the |'| we
+% expected is not there. We fix this as follows: rather than just
+% |::| we set \tn{everyeof} to |::|\Arg{code1} |'::|\Arg{code2}
+% \cs{q_stop}. The auxiliary \cs{@@_set_rescan_single:NNww} runs the
+% \texttt{o}-expanding assignment, expanding either \meta{code1} or
+% \meta{code2} before its the main argument~|#3|. In the typical case
+% without comment character, \meta{code1} is expanded, removing the
+% leading~|'|. In the rarer case with comment character, \meta{code2}
+% is expanded, calling \cs{@@_set_rescan_single_aux:w}, which removes the
+% trailing |::|\Arg{code1} and the leading~|'|.
+% \begin{macrocode}
+\cs_new_protected:Npn \@@_set_rescan:nNN #1
+ {
+ \int_compare:nNnTF \tex_newlinechar:D < 0
+ { \use_ii:nn }
+ {
+ \exp_args:Nnf \tl_if_in:nnTF {#1}
+ { \char_generate:nn { \tex_newlinechar:D } { 12 } }
+ }
+ { \@@_set_rescan_multi:nNN }
+ {
+ \int_set:Nn \tex_endlinechar:D { -1 }
+ \@@_set_rescan_single:nnNN { `' }
+ }
+ {#1}
+ }
+\cs_new_protected:Npn \@@_set_rescan_single:nnNN #1
+ {
+ \int_compare:nNnTF
+ { \char_value_catcode:n {#1} / 2 } = 6
+ {
+ \exp_args:Nof \@@_set_rescan_single_aux:nnnNN
+ \c_@@_rescan_marker_tl
+ { \char_generate:nn {#1} { \char_value_catcode:n {#1} } }
+ }
+ {
+ \int_compare:nNnTF {#1} < { `\~ }
+ {
+ \exp_args:Nf \@@_set_rescan_single:nnNN
+ { \int_eval:n { #1 + 1 } }
+ }
+ { \@@_set_rescan_multi:nNN }
+ }
+ }
+\cs_new_protected:Npn \@@_set_rescan_single_aux:nnnNN #1#2#3#4#5
+ {
+ \tex_everyeof:D
+ {
+ #1 \use_none:n
+ #2 #1 { \exp:w \@@_set_rescan_single_aux:w }
+ \q_stop
+ }
+ \cs_set:Npn \@@_rescan:NNw ##1##2##3 #2 #1 ##4 ##5 \q_stop
+ {
+ \group_end:
+ ##1 ##2 { ##4 ##3 }
+ }
+ \exp_after:wN \@@_rescan:NNw
+ \exp_after:wN #4
+ \exp_after:wN #5
+ \tex_scantokens:D { #2 #3 #2 }
+ }
+\exp_args:Nno \use:nn
+ { \cs_new:Npn \@@_set_rescan_single_aux:w #1 }
+ \c_@@_rescan_marker_tl #2
+ { \use_i:nn \exp_end: #1 }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+%
+% \subsection{Modifying token list variables}
+%
+% \begin{macro}{\tl_replace_all:Nnn, \tl_replace_all:cnn}
+% \begin{macro}{\tl_greplace_all:Nnn, \tl_greplace_all:cnn}
+% \begin{macro}{\tl_replace_once:Nnn, \tl_replace_once:cnn}
+% \begin{macro}{\tl_greplace_once:Nnn, \tl_greplace_once:cnn}
+% All of the \texttt{replace} functions call \cs{@@_replace:NnNNNnn}
+% with appropriate arguments. The first two arguments are explained
+% later. The next controls whether the replacement function calls
+% itself (\cs{@@_replace_next:w}) or stops (\cs{@@_replace_wrap:w})
+% after the first replacement. Next comes an \texttt{x}-type
+% assignment function \cs{tl_set:Nx} or \cs{tl_gset:Nx} for local or
+% global replacements. Finally, the three arguments \meta{tl~var}
+% \Arg{pattern} \Arg{replacement} provided by the user. When
+% describing the auxiliary functions below, we denote the contents of
+% the \meta{tl~var} by \meta{token list}.
+% \begin{macrocode}
+\cs_new_protected:Npn \tl_replace_once:Nnn
+ { \@@_replace:NnNNNnn \q_mark ? \@@_replace_wrap:w \tl_set:Nx }
+\cs_new_protected:Npn \tl_greplace_once:Nnn
+ { \@@_replace:NnNNNnn \q_mark ? \@@_replace_wrap:w \tl_gset:Nx }
+\cs_new_protected:Npn \tl_replace_all:Nnn
+ { \@@_replace:NnNNNnn \q_mark ? \@@_replace_next:w \tl_set:Nx }
+\cs_new_protected:Npn \tl_greplace_all:Nnn
+ { \@@_replace:NnNNNnn \q_mark ? \@@_replace_next:w \tl_gset:Nx }
+\cs_generate_variant:Nn \tl_replace_once:Nnn { c }
+\cs_generate_variant:Nn \tl_greplace_once:Nnn { c }
+\cs_generate_variant:Nn \tl_replace_all:Nnn { c }
+\cs_generate_variant:Nn \tl_greplace_all:Nnn { c }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}
+% {
+% \@@_replace:NnNNNnn,
+% \@@_replace_auxi:NnnNNNnn,
+% \@@_replace_auxii:nNNNnn,
+% \@@_replace_next:w,
+% \@@_replace_wrap:w,
+% }
+% To implement the actual replacement auxiliary
+% \cs{@@_replace_auxii:nNNNnn} we need a \meta{delimiter} with
+% the following properties:
+% \begin{itemize}
+% \item all occurrences of the \meta{pattern}~|#6| in
+% \enquote{\meta{token list} \meta{delimiter}} belong to the
+% \meta{token list} and have no overlap with the \meta{delimiter},
+% \item the first occurrence of the \meta{delimiter} in
+% \enquote{\meta{token list} \meta{delimiter}} is the trailing
+% \meta{delimiter}.
+% \end{itemize}
+% We first find the building blocks for the \meta{delimiter}, namely
+% two tokens \meta{A} and~\meta{B} such that \meta{A} does not appear
+% in~|#6| and |#6| is not~\meta{B} (this condition is trivial if |#6|
+% has more than one token). Then we consider the delimiters
+% \enquote{\meta{A}} and \enquote{\meta{A} \meta{A}$^n$ \meta{B}
+% \meta{A}$^n$ \meta{B}}, for $n\geq 1$, where $\meta{A}^n$ denotes
+% $n$~copies of \meta{A}, and we choose as our \meta{delimiter} the
+% first one which is not in the \meta{token list}.
+%
+% Every delimiter in the set obeys the first condition: |#6|~does not
+% contain~\meta{A} hence cannot be overlapping with the \meta{token
+% list} and the \meta{delimiter}, and it cannot be within the
+% \meta{delimiter} since it would have to be in one of the two
+% \meta{B} hence be equal to this single token (or empty, but this is
+% an error case filtered separately). Given the particular form of
+% these delimiters, for which no prefix is also a suffix, the second
+% condition is actually a consequence of the weaker condition that the
+% \meta{delimiter} we choose does not appear in the \meta{token list}.
+% Additionally, the set of delimiters is such that a \meta{token list}
+% of $n$~tokens can contain at most $O(n^{1/2})$ of them, hence we
+% find a \meta{delimiter} with at most $O(n^{1/2})$ tokens in a time
+% at most $O(n^{3/2})$. Bear in mind that these upper bounds are
+% reached only in very contrived scenarios: we include the case
+% \enquote{\meta{A}} in the list of delimiters to try, so that the
+% \meta{delimiter} is simply \cs{q_mark} in the most common
+% situation where neither the \meta{token list} nor the \meta{pattern}
+% contains \cs{q_mark}.
+%
+% Let us now ahead, optimizing for this most common case. First, two
+% special cases: an empty \meta{pattern}~|#6| is an error, and if
+% |#1|~is absent from both the \meta{token list}~|#5| and the
+% \meta{pattern}~|#6| then we can use it as the \meta{delimiter}
+% through \cs{@@_replace_auxii:nNNNnn} |{#1}|. Otherwise, we end up
+% calling \cs{@@_replace:NnNNNnn} repeatedly with the first two
+% arguments \cs{q_mark} |{?}|, |\?| |{??}|, |\??| |{???}|, and so on,
+% until |#6|~does not contain the control sequence~|#1|, which we take
+% as our~\meta{A}. The argument~|#2| only serves to collect~|?|
+% characters for~|#1|. Note that the order of the tests means that
+% the first two are done every time, which is wasteful (for instance,
+% we repeatedly test for the emptyness of~|#6|). However, this is
+% rare enough not to matter. Finally, choose~\meta{B} to be
+% \cs{q_nil} or~\cs{q_stop} such that it is not equal to~|#6|.
+%
+% The \cs{@@_replace_auxi:NnnNNNnn} auxiliary receives \Arg{A} and
+% |{|\meta{A}$^n$\meta{B}|}| as its arguments, initially with $n=1$.
+% If \enquote{\meta{A} \meta{A}$^n$\meta{B} \meta{A}$^n$\meta{B}} is
+% in the \meta{token list} then increase~$n$ and try again. Once it
+% is not anymore in the \meta{token list} we take it as our
+% \meta{delimiter} and pass this to the \texttt{auxii} auxiliary.
+% \begin{macrocode}
+\cs_new_protected:Npn \@@_replace:NnNNNnn #1#2#3#4#5#6#7
+ {
+ \tl_if_empty:nTF {#6}
+ {
+ \__kernel_msg_error:nnx { kernel } { empty-search-pattern }
+ { \tl_to_str:n {#7} }
+ }
+ {
+ \tl_if_in:onTF { #5 #6 } {#1}
+ {
+ \tl_if_in:nnTF {#6} {#1}
+ { \exp_args:Nc \@@_replace:NnNNNnn {#2} {#2?} }
+ {
+ \quark_if_nil:nTF {#6}
+ { \@@_replace_auxi:NnnNNNnn #5 {#1} { #1 \q_stop } }
+ { \@@_replace_auxi:NnnNNNnn #5 {#1} { #1 \q_nil } }
+ }
+ }
+ { \@@_replace_auxii:nNNNnn {#1} }
+ #3#4#5 {#6} {#7}
+ }
+ }
+\cs_new_protected:Npn \@@_replace_auxi:NnnNNNnn #1#2#3
+ {
+ \tl_if_in:NnTF #1 { #2 #3 #3 }
+ { \@@_replace_auxi:NnnNNNnn #1 { #2 #3 } {#2} }
+ { \@@_replace_auxii:nNNNnn { #2 #3 #3 } }
+ }
+% \end{macrocode}
+% The auxiliary \cs{@@_replace_auxii:nNNNnn} receives the following
+% arguments:
+% \begin{quote}
+% \Arg{delimiter} \meta{function} \meta{assignment} \\
+% \meta{tl~var} \Arg{pattern} \Arg{replacement}
+% \end{quote}
+% All of its work is done between
+% \cs{group_align_safe_begin:} and \cs{group_align_safe_end:} to avoid
+% issues in alignments. It does the actual replacement within
+% |#3|~|#4|~|{...}|, an \texttt{x}-expanding \meta{assignment}~|#3| to
+% the \meta{tl~var}~|#4|. The auxiliary \cs{@@_replace_next:w} is
+% called, followed by the \meta{token list}, some tokens including the
+% \meta{delimiter}~|#1|, followed by the \meta{pattern}~|#5|.
+% This auxiliary finds an argument delimited by~|#5| (the presence of
+% a trailing~|#5| avoids runaway arguments) and calls
+% \cs{@@_replace_wrap:w} to test whether this |#5| is found within the
+% \meta{token list} or is the trailing one.
+%
+% If on the one hand it is found within the \meta{token list}, then
+% |##1| cannot contain the \meta{delimiter}~|#1| that we worked so
+% hard to obtain, thus \cs{@@_replace_wrap:w} gets~|##1| as its own
+% argument~|##1|, and protects it against
+% the \texttt{x}-expanding assignment. It also finds \cs{exp_not:n}
+% as~|##2| and does nothing to it, thus letting through \cs{exp_not:n}
+% \Arg{replacement} into the assignment. Note that
+% \cs{@@_replace_next:w} and \cs{@@_replace_wrap:w} are always called
+% followed by two empty brace groups. These are safe because no
+% delimiter can match them. They prevent losing braces when grabbing
+% delimited arguments, but require the use of \cs{exp_not:o} and
+% \cs{use_none:nn}, rather than simply \cs{exp_not:n}.
+% Afterwards, \cs{@@_replace_next:w} is called
+% to repeat the replacement, or \cs{@@_replace_wrap:w} if we only want
+% a single replacement. In this second case, |##1| is the
+% \meta{remaining tokens} in the \meta{token list} and |##2| is some
+% \meta{ending code} which ends the assignment and removes the
+% trailing tokens |#5| using some \cs{if_false:} |{| \cs{fi:} |}|
+% trickery because~|#5| may contain any delimiter.
+%
+% If on the other hand the argument~|##1| of \cs{@@_replace_next:w} is
+% delimited by the trailing \meta{pattern}~|#5|, then |##1| is
+% \enquote{\{ \} \{ \} \meta{token list} \meta{delimiter}
+% \Arg{ending code}}, hence \cs{@@_replace_wrap:w} finds
+% \enquote{\{ \} \{ \} \meta{token list}} as |##1| and the
+% \meta{ending code} as~|##2|. It leaves the \meta{token list} into
+% the assignment and unbraces the \meta{ending code} which removes
+% what remains (essentially the \meta{delimiter} and
+% \meta{replacement}).
+% \begin{macrocode}
+\cs_new_protected:Npn \@@_replace_auxii:nNNNnn #1#2#3#4#5#6
+ {
+ \group_align_safe_begin:
+ \cs_set:Npn \@@_replace_wrap:w ##1 #1 ##2
+ { \exp_not:o { \use_none:nn ##1 } ##2 }
+ \cs_set:Npx \@@_replace_next:w ##1 #5
+ {
+ \exp_not:N \@@_replace_wrap:w ##1
+ \exp_not:n { #1 }
+ \exp_not:n { \exp_not:n {#6} }
+ \exp_not:n { #2 { } { } }
+ }
+ #3 #4
+ {
+ \exp_after:wN \@@_replace_next:w
+ \exp_after:wN { \exp_after:wN }
+ \exp_after:wN { \exp_after:wN }
+ #4
+ #1
+ {
+ \if_false: { \fi: }
+ \exp_after:wN \use_none:n \exp_after:wN { \if_false: } \fi:
+ }
+ #5
+ }
+ \group_align_safe_end:
+ }
+\cs_new_eq:NN \@@_replace_wrap:w ?
+\cs_new_eq:NN \@@_replace_next:w ?
+% \end{macrocode}
+% \end{macro}
+%
+% \begin{macro}{\tl_remove_once:Nn, \tl_remove_once:cn}
+% \begin{macro}{\tl_gremove_once:Nn, \tl_gremove_once:cn}
+% Removal is just a special case of replacement.
+% \begin{macrocode}
+\cs_new_protected:Npn \tl_remove_once:Nn #1#2
+ { \tl_replace_once:Nnn #1 {#2} { } }
+\cs_new_protected:Npn \tl_gremove_once:Nn #1#2
+ { \tl_greplace_once:Nnn #1 {#2} { } }
+\cs_generate_variant:Nn \tl_remove_once:Nn { c }
+\cs_generate_variant:Nn \tl_gremove_once:Nn { c }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}{\tl_remove_all:Nn, \tl_remove_all:cn}
+% \begin{macro}{\tl_gremove_all:Nn, \tl_gremove_all:cn}
+% Removal is just a special case of replacement.
+% \begin{macrocode}
+\cs_new_protected:Npn \tl_remove_all:Nn #1#2
+ { \tl_replace_all:Nnn #1 {#2} { } }
+\cs_new_protected:Npn \tl_gremove_all:Nn #1#2
+ { \tl_greplace_all:Nnn #1 {#2} { } }
+\cs_generate_variant:Nn \tl_remove_all:Nn { c }
+\cs_generate_variant:Nn \tl_gremove_all:Nn { c }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \subsection{Token list conditionals}
+%
+% \begin{macro}[pTF]{\tl_if_blank:n, \tl_if_blank:V, \tl_if_blank:o}
+% \begin{macro}{\@@_if_blank_p:NNw}
+% \TeX{} skips spaces when reading a non-delimited arguments. Thus,
+% a \meta{token list} is blank if and only if \cs{use_none:n}
+% \meta{token list} |?| is empty after one expansion. The auxiliary
+% \cs{@@_if_empty_if:o} is a fast emptyness test, converting its
+% argument to a string (after one expansion) and using the test
+% \cs{if_meaning:w} \cs{q_nil} |...| \cs{q_nil}.
+% \begin{macrocode}
+\prg_new_conditional:Npnn \tl_if_blank:n #1 { p , T , F , TF }
+ {
+ \@@_if_empty_if:o { \use_none:n #1 ? }
+ \prg_return_true:
+ \else:
+ \prg_return_false:
+ \fi:
+ }
+\prg_generate_conditional_variant:Nnn \tl_if_blank:n
+ { V , o } { p , T , F , TF }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}[pTF]{\tl_if_empty:N, \tl_if_empty:c}
+% These functions check whether the token list in the argument is
+% empty and execute the proper code from their argument(s).
+% \begin{macrocode}
+\prg_new_conditional:Npnn \tl_if_empty:N #1 { p , T , F , TF }
+ {
+ \if_meaning:w #1 \c_empty_tl
+ \prg_return_true:
+ \else:
+ \prg_return_false:
+ \fi:
+ }
+\prg_generate_conditional_variant:Nnn \tl_if_empty:N
+ { c } { p , T , F , TF }
+% \end{macrocode}
+% \end{macro}
+%
+% \begin{macro}[pTF]{\tl_if_empty:n, \tl_if_empty:V}
+% Convert the argument to a string: this is empty if and only if
+% the argument is. Then |\if_meaning:w \q_nil ... \q_nil| is
+% \texttt{true} if and only if the string |...| is empty.
+% It could be tempting to use |\if_meaning:w \q_nil #1 \q_nil| directly.
+% This fails on a token
+% list starting with \cs{q_nil} of course but more troubling is the
+% case where argument is a complete conditional such as \cs{if_true:}
+% a \cs{else:} b \cs{fi:} because then \cs{if_true:} is used by
+% \cs{if_meaning:w}, the test turns out \texttt{false}, the \cs{else:}
+% executes the \texttt{false} branch, the \cs{fi:} ends it and the
+% \cs{q_nil} at the end
+% starts executing\dots{}
+% \begin{macrocode}
+\prg_new_conditional:Npnn \tl_if_empty:n #1 { p , TF , T , F }
+ {
+ \exp_after:wN \if_meaning:w \exp_after:wN \q_nil
+ \tl_to_str:n {#1} \q_nil
+ \prg_return_true:
+ \else:
+ \prg_return_false:
+ \fi:
+ }
+\prg_generate_conditional_variant:Nnn \tl_if_empty:n
+ { V } { p , TF , T , F }
+% \end{macrocode}
+% \end{macro}
+%
+% \begin{macro}[pTF,documented-as=\tl_if_empty:nTF]{\tl_if_empty:o}
+% \begin{macro}[EXP]{\@@_if_empty_if:o}
+% The auxiliary function \cs{@@_if_empty_if:o} is for use
+% in various token list conditionals which reduce to testing
+% if a given token list is empty after applying a simple function
+% to it.
+% The test for emptiness is based on \cs{tl_if_empty:nTF}, but
+% the expansion is hard-coded for efficiency, as this auxiliary
+% function is used in several places.
+% We don't put \cs{prg_return_true:} and so on in the definition of
+% the auxiliary, because that would prevent an optimization applied to
+% conditionals that end with this code.
+% \begin{macrocode}
+\cs_new:Npn \@@_if_empty_if:o #1
+ {
+ \exp_after:wN \if_meaning:w \exp_after:wN \q_nil
+ \__kernel_tl_to_str:w \exp_after:wN {#1} \q_nil
+ }
+\prg_new_conditional:Npnn \tl_if_empty:o #1 { p , TF , T , F }
+ {
+ \@@_if_empty_if:o {#1}
+ \prg_return_true:
+ \else:
+ \prg_return_false:
+ \fi:
+ }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}[pTF]{\tl_if_eq:NN, \tl_if_eq:Nc, \tl_if_eq:cN, \tl_if_eq:cc}
+% Returns \cs{c_true_bool} if and only if the two token list variables are
+% equal.
+% \begin{macrocode}
+\prg_new_conditional:Npnn \tl_if_eq:NN #1#2 { p , T , F , TF }
+ {
+ \if_meaning:w #1 #2
+ \prg_return_true:
+ \else:
+ \prg_return_false:
+ \fi:
+ }
+\prg_generate_conditional_variant:Nnn \tl_if_eq:NN
+ { Nc , c , cc } { p , TF , T , F }
+% \end{macrocode}
+% \end{macro}
+%
+% \begin{macro}[TF]{\tl_if_eq:nn}
+% \begin{variable}{\l_@@_internal_a_tl, \l_@@_internal_b_tl}
+% A simple store and compare routine.
+% \begin{macrocode}
+\prg_new_protected_conditional:Npnn \tl_if_eq:nn #1#2 { T , F , TF }
+ {
+ \group_begin:
+ \tl_set:Nn \l_@@_internal_a_tl {#1}
+ \tl_set:Nn \l_@@_internal_b_tl {#2}
+ \exp_after:wN
+ \group_end:
+ \if_meaning:w \l_@@_internal_a_tl \l_@@_internal_b_tl
+ \prg_return_true:
+ \else:
+ \prg_return_false:
+ \fi:
+ }
+\tl_new:N \l_@@_internal_a_tl
+\tl_new:N \l_@@_internal_b_tl
+% \end{macrocode}
+% \end{variable}
+% \end{macro}
+%
+% \begin{macro}[TF]{\tl_if_in:Nn, \tl_if_in:cn}
+% See \cs{tl_if_in:nnTF} for further comments. Here we simply
+% expand the token list variable and pass it to \cs{tl_if_in:nnTF}.
+% \begin{macrocode}
+\cs_new_protected:Npn \tl_if_in:NnT { \exp_args:No \tl_if_in:nnT }
+\cs_new_protected:Npn \tl_if_in:NnF { \exp_args:No \tl_if_in:nnF }
+\cs_new_protected:Npn \tl_if_in:NnTF { \exp_args:No \tl_if_in:nnTF }
+\prg_generate_conditional_variant:Nnn \tl_if_in:Nn
+ { c } { T , F , TF }
+% \end{macrocode}
+% \end{macro}
+%
+% \begin{macro}[TF]{\tl_if_in:nn, \tl_if_in:Vn, \tl_if_in:on, \tl_if_in:no}
+% Once more, the test relies on the emptiness test for robustness.
+% The function \cs{@@_tmp:w} removes tokens until the first occurrence
+% of |#2|. If this does not appear in |#1|, then the final |#2| is removed,
+% leaving an empty token list. Otherwise some tokens remain, and the
+% test is \texttt{false}. See \cs{tl_if_empty:nTF} for details on
+% the emptiness test.
+%
+% Treating correctly cases like
+% |\tl_if_in:nnTF {a state}{states}|, where |#1#2| contains |#2| before
+% the end, requires special care.
+% To cater for this case, we insert |{}{}| between the two token
+% lists. This marker may not appear in |#2| because of \TeX{} limitations
+% on what can delimit a parameter, hence we are safe. Using two brace
+% groups makes the test work also for empty arguments.
+% The \cs{if_false:} constructions are a faster way to do
+% \cs{group_align_safe_begin:} and \cs{group_align_safe_end:}.
+% The \cs{scan_stop:} ensures that \texttt{f}-expanding
+% \cs{tl_if_in:nn} does not lead to unbalanced braces.
+% \begin{macrocode}
+\prg_new_protected_conditional:Npnn \tl_if_in:nn #1#2 { T , F , TF }
+ {
+ \scan_stop:
+ \if_false: { \fi:
+ \cs_set:Npn \@@_tmp:w ##1 #2 { }
+ \tl_if_empty:oTF { \@@_tmp:w #1 {} {} #2 }
+ { \prg_return_false: } { \prg_return_true: }
+ \if_false: } \fi:
+ }
+\prg_generate_conditional_variant:Nnn \tl_if_in:nn
+ { V , o , no } { T , F , TF }
+% \end{macrocode}
+% \end{macro}
+%
+% \begin{macro}[pTF, EXP]{\tl_if_novalue:n}
+% \begin{macro}[EXP]{\@@_if_novalue:w}
+% Tests for |-NoValue-|: this is similar to \cs{tl_if_in:nn} but set
+% up to be expandable and to check the value exactly. The question
+% mark prevents the auxiliary from losing braces.
+% \begin{macrocode}
+\cs_set_protected:Npn \@@_tmp:w #1
+ {
+ \prg_new_conditional:Npnn \tl_if_novalue:n ##1
+ { p , T , F , TF }
+ {
+ \str_if_eq:onTF
+ { \@@_if_novalue:w ? ##1 { } #1 }
+ { ? { } #1 }
+ { \prg_return_true: }
+ { \prg_return_false: }
+ }
+ \cs_new:Npn \@@_if_novalue:w ##1 #1 {##1}
+ }
+\exp_args:No \@@_tmp:w { \c_novalue_tl }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}[EXP,pTF]{\tl_if_single:N}
+% Expand the token list and feed it to \cs{tl_if_single:n}.
+% \begin{macrocode}
+\cs_new:Npn \tl_if_single_p:N { \exp_args:No \tl_if_single_p:n }
+\cs_new:Npn \tl_if_single:NT { \exp_args:No \tl_if_single:nT }
+\cs_new:Npn \tl_if_single:NF { \exp_args:No \tl_if_single:nF }
+\cs_new:Npn \tl_if_single:NTF { \exp_args:No \tl_if_single:nTF }
+% \end{macrocode}
+% \end{macro}
+%
+% \begin{macro}[EXP,pTF]{\tl_if_single:n}
+% \begin{macro}[EXP,pTF]{\@@_if_single:n}
+% This test is similar to \cs{tl_if_empty:nTF}. Expanding
+% \cs{use_none:nn} |#1| |??| once yields an empty result if |#1| is
+% blank, a single~|?| if |#1| has a single item, and otherwise yields
+% some tokens ending with |??|. Then, \cs{tl_to_str:n} makes sure
+% there are no odd category codes. An earlier version would compare
+% the result to a single~|?| using string comparison, but the Lua call
+% is slow in \LuaTeX{}. Instead, \cs{@@_if_single:nnw} picks the
+% second token in front of it. If |#1| is empty, this token is
+% the trailing~|?| and the catcode test yields \texttt{false}. If
+% |#1| has a single item, the token is~|^| and the catcode test
+% yields \texttt{true}. Otherwise, it is one of the characters
+% resulting from \cs{tl_to_str:n}, and the catcode test yields
+% \texttt{false}. Note that \cs{if_catcode:w} and
+% \cs{__kernel_tl_to_str:w} are primitives that take care of
+% expansion.
+% \begin{macrocode}
+\prg_new_conditional:Npnn \tl_if_single:n #1 { p , T , F , TF }
+ {
+ \if_catcode:w ^ \exp_after:wN \@@_if_single:nnw
+ \__kernel_tl_to_str:w
+ \exp_after:wN { \use_none:nn #1 ?? } ^ ? \q_stop
+ \prg_return_true:
+ \else:
+ \prg_return_false:
+ \fi:
+ }
+\cs_new:Npn \@@_if_single:nnw #1#2#3 \q_stop {#2}
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}[EXP,pTF]{\tl_if_single_token:n}
+% There are four cases: empty token list, token list starting with a
+% normal token, with a brace group, or with a space token. If the
+% token list starts with a normal token, remove it and check for
+% emptiness. For the next case, an empty token list is not a single
+% token. Finally, we have a non-empty token list starting with a
+% space or a brace group. Applying \texttt{f}-expansion yields an
+% empty result if and only if the token list is a single space.
+% \begin{macrocode}
+\prg_new_conditional:Npnn \tl_if_single_token:n #1 { p , T , F , TF }
+ {
+ \tl_if_head_is_N_type:nTF {#1}
+ { \@@_if_empty_if:o { \use_none:n #1 } }
+ {
+ \tl_if_empty:nTF {#1}
+ { \if_false: }
+ { \@@_if_empty_if:o { \exp:w \exp_end_continue_f:w #1 } }
+ }
+ \prg_return_true:
+ \else:
+ \prg_return_false:
+ \fi:
+ }
+% \end{macrocode}
+% \end{macro}
+%
+% \begin{macro}[EXP, noTF]{\tl_case:Nn, \tl_case:cn}
+% \begin{macro}[EXP]{\@@_case:nnTF}
+% \begin{macro}[EXP]{\@@_case:Nw}
+% \begin{macro}[EXP]{\@@_case_end:nw}
+% The aim here is to allow the case statement to be evaluated
+% using a known number of expansion steps (two), and without
+% needing to use an explicit \enquote{end of recursion} marker.
+% That is achieved by using the test input as the final case,
+% as this is always true. The trick is then to tidy up
+% the output such that the appropriate case code plus either
+% the \texttt{true} or \texttt{false} branch code is inserted.
+% \begin{macrocode}
+\cs_new:Npn \tl_case:Nn #1#2
+ {
+ \exp:w
+ \@@_case:NnTF #1 {#2} { } { }
+ }
+\cs_new:Npn \tl_case:NnT #1#2#3
+ {
+ \exp:w
+ \@@_case:NnTF #1 {#2} {#3} { }
+ }
+\cs_new:Npn \tl_case:NnF #1#2#3
+ {
+ \exp:w
+ \@@_case:NnTF #1 {#2} { } {#3}
+ }
+\cs_new:Npn \tl_case:NnTF #1#2
+ {
+ \exp:w
+ \@@_case:NnTF #1 {#2}
+ }
+\cs_new:Npn \@@_case:NnTF #1#2#3#4
+ { \@@_case:Nw #1 #2 #1 { } \q_mark {#3} \q_mark {#4} \q_stop }
+\cs_new:Npn \@@_case:Nw #1#2#3
+ {
+ \tl_if_eq:NNTF #1 #2
+ { \@@_case_end:nw {#3} }
+ { \@@_case:Nw #1 }
+ }
+\cs_generate_variant:Nn \tl_case:Nn { c }
+\prg_generate_conditional_variant:Nnn \tl_case:Nn
+ { c } { T , F , TF }
+% \end{macrocode}
+% To tidy up the recursion, there are two outcomes. If there was a hit to
+% one of the cases searched for, then |#1| is the code to insert,
+% |#2| is the \emph{next} case to check on and |#3| is all of
+% the rest of the cases code. That means that |#4| is the \texttt{true}
+% branch code, and |#5| tidies up the spare \cs{q_mark} and the
+% \texttt{false} branch. On the other hand, if none of the cases matched
+% then we arrive here using the \enquote{termination} case of comparing
+% the search with itself. That means that |#1| is empty, |#2| is
+% the first \cs{q_mark} and so |#4| is the \texttt{false} code (the
+% \texttt{true} code is mopped up by |#3|).
+% \begin{macrocode}
+\cs_new:Npn \@@_case_end:nw #1#2#3 \q_mark #4#5 \q_stop
+ { \exp_end: #1 #4 }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+%
+% \subsection{Mapping to token lists}
+%
+% \begin{macro}{\tl_map_function:nN}
+% \begin{macro}{\tl_map_function:NN, \tl_map_function:cN}
+% \begin{macro}{\@@_map_function:Nn}
+% Expandable loop macro for token lists. These have the advantage of not
+% needing to test if the argument is empty, because if it is, the stop
+% marker is read immediately and the loop terminated.
+% \begin{macrocode}
+\cs_new:Npn \tl_map_function:nN #1#2
+ {
+ \@@_map_function:Nn #2 #1
+ \q_recursion_tail
+ \prg_break_point:Nn \tl_map_break: { }
+ }
+\cs_new:Npn \tl_map_function:NN
+ { \exp_args:No \tl_map_function:nN }
+\cs_new:Npn \@@_map_function:Nn #1#2
+ {
+ \quark_if_recursion_tail_break:nN {#2} \tl_map_break:
+ #1 {#2} \@@_map_function:Nn #1
+ }
+\cs_generate_variant:Nn \tl_map_function:NN { c }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}{\tl_map_inline:nn}
+% \begin{macro}{\tl_map_inline:Nn, \tl_map_inline:cn}
+% The inline functions are straight forward by now. We use a little
+% trick with the counter \cs{g__kernel_prg_map_int} to make
+% them nestable. We can also make use of \cs{@@_map_function:Nn}
+% from before.
+% \begin{macrocode}
+\cs_new_protected:Npn \tl_map_inline:nn #1#2
+ {
+ \int_gincr:N \g__kernel_prg_map_int
+ \cs_gset_protected:cpn
+ { @@_map_ \int_use:N \g__kernel_prg_map_int :w } ##1 {#2}
+ \exp_args:Nc \@@_map_function:Nn
+ { @@_map_ \int_use:N \g__kernel_prg_map_int :w }
+ #1 \q_recursion_tail
+ \prg_break_point:Nn \tl_map_break:
+ { \int_gdecr:N \g__kernel_prg_map_int }
+ }
+\cs_new_protected:Npn \tl_map_inline:Nn
+ { \exp_args:No \tl_map_inline:nn }
+\cs_generate_variant:Nn \tl_map_inline:Nn { c }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}{\tl_map_variable:nNn}
+% \begin{macro}{\tl_map_variable:NNn, \tl_map_variable:cNn}
+% \begin{macro}{\@@_map_variable:Nnn}
+% \cs{tl_map_variable:nNn} \meta{token list} \meta{tl~var}
+% \meta{action} assigns \meta{tl~var} to each element and executes
+% \meta{action}. The assignment to \meta{tl~var} is done after the
+% quark test so that this variable does not get set to a quark.
+% \begin{macrocode}
+\cs_new_protected:Npn \tl_map_variable:nNn #1#2#3
+ {
+ \@@_map_variable:Nnn #2 {#3} #1
+ \q_recursion_tail
+ \prg_break_point:Nn \tl_map_break: { }
+ }
+\cs_new_protected:Npn \tl_map_variable:NNn
+ { \exp_args:No \tl_map_variable:nNn }
+\cs_new_protected:Npn \@@_map_variable:Nnn #1#2#3
+ {
+ \quark_if_recursion_tail_break:nN {#3} \tl_map_break:
+ \tl_set:Nn #1 {#3}
+ \use:n {#2}
+ \@@_map_variable:Nnn #1 {#2}
+ }
+\cs_generate_variant:Nn \tl_map_variable:NNn { c }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}{\tl_map_break:}
+% \begin{macro}{\tl_map_break:n}
+% The break statements use the general \cs{prg_map_break:Nn}.
+% \begin{macrocode}
+\cs_new:Npn \tl_map_break:
+ { \prg_map_break:Nn \tl_map_break: { } }
+\cs_new:Npn \tl_map_break:n
+ { \prg_map_break:Nn \tl_map_break: }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \subsection{Using token lists}
+%
+% \begin{macro}{\tl_to_str:n, \tl_to_str:V}
+% Another name for a primitive: defined in \pkg{l3basics}.
+% \begin{macrocode}
+\cs_generate_variant:Nn \tl_to_str:n { V }
+% \end{macrocode}
+% \end{macro}
+%
+% \begin{macro}{\tl_to_str:N, \tl_to_str:c}
+% These functions return the replacement text of a token list as a
+% string.
+% \begin{macrocode}
+\cs_new:Npn \tl_to_str:N #1 { \__kernel_tl_to_str:w \exp_after:wN {#1} }
+\cs_generate_variant:Nn \tl_to_str:N { c }
+% \end{macrocode}
+% \end{macro}
+%
+% \begin{macro}{\tl_use:N, \tl_use:c}
+% Token lists which are simply not defined give a clear \TeX{}
+% error here. No such luck for ones equal to \cs{scan_stop:} so
+% instead a test is made and if there is an issue an error is forced.
+% \begin{macrocode}
+\cs_new:Npn \tl_use:N #1
+ {
+ \tl_if_exist:NTF #1 {#1}
+ {
+ \__kernel_msg_expandable_error:nnn
+ { kernel } { bad-variable } {#1}
+ }
+ }
+\cs_generate_variant:Nn \tl_use:N { c }
+% \end{macrocode}
+% \end{macro}
+%
+% \subsection{Working with the contents of token lists}
+%
+% \begin{macro}{\tl_count:n, \tl_count:V, \tl_count:o}
+% \begin{macro}{\tl_count:N, \tl_count:c}
+% \begin{macro}{\@@_count:n}
+% Count number of elements within a token list or token list
+% variable. Brace groups within the list are read as a single
+% element. Spaces are ignored.
+% \cs{@@_count:n} grabs the element and replaces it by |+1|.
+% The |0| ensures that it works on an empty list.
+% \begin{macrocode}
+\cs_new:Npn \tl_count:n #1
+ {
+ \int_eval:n
+ { 0 \tl_map_function:nN {#1} \@@_count:n }
+ }
+\cs_new:Npn \tl_count:N #1
+ {
+ \int_eval:n
+ { 0 \tl_map_function:NN #1 \@@_count:n }
+ }
+\cs_new:Npn \@@_count:n #1 { + 1 }
+\cs_generate_variant:Nn \tl_count:n { V , o }
+\cs_generate_variant:Nn \tl_count:N { c }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+%
+%
+% \begin{macro}[EXP]{\tl_count_tokens:n}
+% \begin{macro}[EXP]{\@@_act_count_normal:nN,
+% \@@_act_count_group:nn, \@@_act_count_space:n}
+% The token count is computed through an \cs{int_eval:n} construction.
+% Each \texttt{1+} is output to the \emph{left}, into the integer
+% expression, and the sum is ended by the \cs{exp_end:} inserted by
+% \cs{@@_act_end:wn} (which is technically implemented as \cs{c_zero_int}).
+% Somewhat a hack!
+% \begin{macrocode}
+\cs_new:Npn \tl_count_tokens:n #1
+ {
+ \int_eval:n
+ {
+ \@@_act:NNNnn
+ \@@_act_count_normal:nN
+ \@@_act_count_group:nn
+ \@@_act_count_space:n
+ { }
+ {#1}
+ }
+ }
+\cs_new:Npn \@@_act_count_normal:nN #1 #2 { 1 + }
+\cs_new:Npn \@@_act_count_space:n #1 { 1 + }
+\cs_new:Npn \@@_act_count_group:nn #1 #2
+ { 2 + \tl_count_tokens:n {#2} + }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}{\tl_reverse_items:n}
+% \begin{macro}{\@@_reverse_items:nwNwn}
+% \begin{macro}{\@@_reverse_items:wn}
+% Reversal of a token list is done by taking one item at a time
+% and putting it after \cs{q_stop}.
+% \begin{macrocode}
+\cs_new:Npn \tl_reverse_items:n #1
+ {
+ \@@_reverse_items:nwNwn #1 ?
+ \q_mark \@@_reverse_items:nwNwn
+ \q_mark \@@_reverse_items:wn
+ \q_stop { }
+ }
+\cs_new:Npn \@@_reverse_items:nwNwn #1 #2 \q_mark #3 #4 \q_stop #5
+ {
+ #3 #2
+ \q_mark \@@_reverse_items:nwNwn
+ \q_mark \@@_reverse_items:wn
+ \q_stop { {#1} #5 }
+ }
+\cs_new:Npn \@@_reverse_items:wn #1 \q_stop #2
+ { \exp_not:o { \use_none:nn #2 } }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}{\tl_trim_spaces:n, \tl_trim_spaces:o}
+% \begin{macro}{\tl_trim_spaces_apply:nN, \tl_trim_spaces_apply:oN}
+% \begin{macro}
+% {
+% \tl_trim_spaces:N, \tl_trim_spaces:c,
+% \tl_gtrim_spaces:N, \tl_gtrim_spaces:c
+% }
+% Trimming spaces from around the input is deferred to an internal
+% function whose first argument is the token list to trim, augmented
+% by an initial \cs{q_mark}, and whose second argument is a
+% \meta{continuation}, which receives as a braced argument
+% \cs{use_none:n} \cs{q_mark} \meta{trimmed token list}. In the case
+% at hand, we take \cs{exp_not:o} as our continuation, so that space
+% trimming behaves correctly within an \texttt{x}-type expansion.
+% \begin{macrocode}
+\cs_new:Npn \tl_trim_spaces:n #1
+ { \@@_trim_spaces:nn { \q_mark #1 } \exp_not:o }
+\cs_generate_variant:Nn \tl_trim_spaces:n { o }
+\cs_new:Npn \tl_trim_spaces_apply:nN #1#2
+ { \@@_trim_spaces:nn { \q_mark #1 } { \exp_args:No #2 } }
+\cs_generate_variant:Nn \tl_trim_spaces_apply:nN { o }
+\cs_new_protected:Npn \tl_trim_spaces:N #1
+ { \tl_set:Nx #1 { \exp_args:No \tl_trim_spaces:n {#1} } }
+\cs_new_protected:Npn \tl_gtrim_spaces:N #1
+ { \tl_gset:Nx #1 { \exp_args:No \tl_trim_spaces:n {#1} } }
+\cs_generate_variant:Nn \tl_trim_spaces:N { c }
+\cs_generate_variant:Nn \tl_gtrim_spaces:N { c }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}{\@@_trim_spaces:nn}
+% \begin{macro}
+% {
+% \@@_trim_spaces_auxi:w, \@@_trim_spaces_auxii:w,
+% \@@_trim_spaces_auxiii:w, \@@_trim_spaces_auxiv:w
+% }
+% Trimming spaces from around the input is done using delimited
+% arguments and quarks, and to get spaces at odd places in the
+% definitions, we nest those in \cs{@@_tmp:w}, which then receives
+% a single space as its argument: |#1| is \verb*+ +.
+% Removing leading spaces is done with \cs{@@_trim_spaces_auxi:w},
+% which loops until \cs{q_mark}\verb*+ + matches the end of the token
+% list: then |##1| is the token list and |##3| is
+% \cs{@@_trim_spaces_auxii:w}. This hands the relevant tokens to the
+% loop \cs{@@_trim_spaces_auxiii:w}, responsible for trimming
+% trailing spaces. The end is reached when \verb*+ + \cs{q_nil}
+% matches the one present in the definition of \cs{tl_trim_spacs:n}.
+% Then \cs{@@_trim_spaces_auxiv:w} puts the token list into a group,
+% with \cs{use_none:n} placed there to gobble a lingering \cs{q_mark},
+% and feeds this to the \meta{continuation}.
+% \begin{macrocode}
+\cs_set:Npn \@@_tmp:w #1
+ {
+ \cs_new:Npn \@@_trim_spaces:nn ##1
+ {
+ \@@_trim_spaces_auxi:w
+ ##1
+ \q_nil
+ \q_mark #1 { }
+ \q_mark \@@_trim_spaces_auxii:w
+ \@@_trim_spaces_auxiii:w
+ #1 \q_nil
+ \@@_trim_spaces_auxiv:w
+ \q_stop
+ }
+ \cs_new:Npn \@@_trim_spaces_auxi:w ##1 \q_mark #1 ##2 \q_mark ##3
+ {
+ ##3
+ \@@_trim_spaces_auxi:w
+ \q_mark
+ ##2
+ \q_mark #1 {##1}
+ }
+ \cs_new:Npn \@@_trim_spaces_auxii:w
+ \@@_trim_spaces_auxi:w \q_mark \q_mark ##1
+ {
+ \@@_trim_spaces_auxiii:w
+ ##1
+ }
+ \cs_new:Npn \@@_trim_spaces_auxiii:w ##1 #1 \q_nil ##2
+ {
+ ##2
+ ##1 \q_nil
+ \@@_trim_spaces_auxiii:w
+ }
+ \cs_new:Npn \@@_trim_spaces_auxiv:w ##1 \q_nil ##2 \q_stop ##3
+ { ##3 { \use_none:n ##1 } }
+ }
+\@@_tmp:w { ~ }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}
+% {\tl_sort:Nn, \tl_sort:cn, \tl_gsort:Nn, \tl_gsort:cn, \tl_sort:nN}
+% Implemented in \pkg{l3sort}.
+% \end{macro}
+%
+% \subsection{Token by token changes}
+%
+% \begin{variable}{\q_@@_act_mark, \q_@@_act_stop}
+% The \cs[no-index]{@@_act_\ldots{}} functions may be applied to any token list.
+% Hence, we use two private quarks, to allow any token, even quarks,
+% in the token list.
+% Only \cs{q_@@_act_mark} and \cs{q_@@_act_stop} may not appear
+% in the token lists manipulated by \cs{@@_act:NNNnn} functions.
+% No quark module yet, so do things by hand.
+% \begin{macrocode}
+\cs_new_nopar:Npn \q_@@_act_mark { \q_@@_act_mark }
+\cs_new_nopar:Npn \q_@@_act_stop { \q_@@_act_stop }
+% \end{macrocode}
+% \end{variable}
+%
+% \begin{macro}[EXP]{\@@_act:NNNnn}
+% \begin{macro}[EXP]{\@@_act_output:n, \@@_act_reverse_output:n}
+% \begin{macro}[EXP]{\@@_act_loop:w}
+% \begin{macro}[EXP]{\@@_act_normal:NwnNNN}
+% \begin{macro}[EXP]{\@@_act_group:nwnNNN}
+% \begin{macro}[EXP]{\@@_act_space:wwnNNN}
+% \begin{macro}[EXP]{\@@_act_end:w}
+% To help control the expansion, \cs{@@_act:NNNnn} should always
+% be proceeded by \cs{exp:w} and ends by producing \cs{exp_end:}
+% once the result has been obtained. Then loop over tokens,
+% groups, and spaces in |#5|. The marker \cs{q_@@_act_mark}
+% is used both to avoid losing outer braces and to detect the
+% end of the token list more easily. The result is stored
+% as an argument for the dummy function \cs{@@_act_result:n}.
+% \begin{macrocode}
+\cs_new:Npn \@@_act:NNNnn #1#2#3#4#5
+ {
+ \group_align_safe_begin:
+ \@@_act_loop:w #5 \q_@@_act_mark \q_@@_act_stop
+ {#4} #1 #2 #3
+ \@@_act_result:n { }
+ }
+% \end{macrocode}
+% In the loop, we check how the token list begins and act
+% accordingly. In the \enquote{normal} case, we may have
+% reached \cs{q_@@_act_mark}, the end of the list. Then
+% leave \cs{exp_end:} and the result in the input stream,
+% to terminate the expansion of \cs{exp:w}.
+% Otherwise, apply the relevant function to the
+% \enquote{arguments}, |#3|
+% and to the head of the token list. Then repeat the loop.
+% The scheme is the same if the token list starts with a
+% group or with a space. Some extra work is needed to
+% make \cs{@@_act_space:wwnNNN} gobble the space.
+% \begin{macrocode}
+\cs_new:Npn \@@_act_loop:w #1 \q_@@_act_stop
+ {
+ \tl_if_head_is_N_type:nTF {#1}
+ { \@@_act_normal:NwnNNN }
+ {
+ \tl_if_head_is_group:nTF {#1}
+ { \@@_act_group:nwnNNN }
+ { \@@_act_space:wwnNNN }
+ }
+ #1 \q_@@_act_stop
+ }
+\cs_new:Npn \@@_act_normal:NwnNNN #1 #2 \q_@@_act_stop #3#4
+ {
+ \if_meaning:w \q_@@_act_mark #1
+ \exp_after:wN \@@_act_end:wn
+ \fi:
+ #4 {#3} #1
+ \@@_act_loop:w #2 \q_@@_act_stop
+ {#3} #4
+ }
+\cs_new:Npn \@@_act_end:wn #1 \@@_act_result:n #2
+ { \group_align_safe_end: \exp_end: #2 }
+\cs_new:Npn \@@_act_group:nwnNNN #1 #2 \q_@@_act_stop #3#4#5
+ {
+ #5 {#3} {#1}
+ \@@_act_loop:w #2 \q_@@_act_stop
+ {#3} #4 #5
+ }
+\exp_last_unbraced:NNo
+ \cs_new:Npn \@@_act_space:wwnNNN \c_space_tl #1 \q_@@_act_stop #2#3#4#5
+ {
+ #5 {#2}
+ \@@_act_loop:w #1 \q_@@_act_stop
+ {#2} #3 #4 #5
+ }
+% \end{macrocode}
+% Typically, the output is done to the right of what was already output,
+% using \cs{@@_act_output:n}, but for the \cs{@@_act_reverse} functions,
+% it should be done to the left.
+% \begin{macrocode}
+\cs_new:Npn \@@_act_output:n #1 #2 \@@_act_result:n #3
+ { #2 \@@_act_result:n { #3 #1 } }
+\cs_new:Npn \@@_act_reverse_output:n #1 #2 \@@_act_result:n #3
+ { #2 \@@_act_result:n { #1 #3 } }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}[EXP]{\tl_reverse:n, \tl_reverse:o, \tl_reverse:V}
+% \begin{macro}[EXP]{\@@_reverse_normal:nN}
+% \begin{macro}[EXP]{\@@_reverse_group_preserve:nn}
+% \begin{macro}[EXP]{\@@_reverse_space:n}
+% The goal here is to reverse without losing spaces nor braces.
+% This is done using the general internal function \cs{@@_act:NNNnn}.
+% Spaces and \enquote{normal} tokens are output on the left of the current
+% output. Grouped tokens are output to the left but without any reversal
+% within the group. All of the internal functions here drop one argument:
+% this is needed by \cs{@@_act:NNNnn} when changing case (to record
+% which direction the change is in), but not when reversing the tokens.
+% \begin{macrocode}
+\cs_new:Npn \tl_reverse:n #1
+ {
+ \__kernel_exp_not:w \exp_after:wN
+ {
+ \exp:w
+ \@@_act:NNNnn
+ \@@_reverse_normal:nN
+ \@@_reverse_group_preserve:nn
+ \@@_reverse_space:n
+ { }
+ {#1}
+ }
+ }
+\cs_generate_variant:Nn \tl_reverse:n { o , V }
+\cs_new:Npn \@@_reverse_normal:nN #1#2
+ { \@@_act_reverse_output:n {#2} }
+\cs_new:Npn \@@_reverse_group_preserve:nn #1#2
+ { \@@_act_reverse_output:n { {#2} } }
+\cs_new:Npn \@@_reverse_space:n #1
+ { \@@_act_reverse_output:n { ~ } }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}{\tl_reverse:N, \tl_reverse:c, \tl_greverse:N, \tl_greverse:c}
+% This reverses the list, leaving \cs{exp_stop_f:} in front,
+% which stops the \texttt{f}-expansion.
+% \begin{macrocode}
+\cs_new_protected:Npn \tl_reverse:N #1
+ { \tl_set:Nx #1 { \exp_args:No \tl_reverse:n { #1 } } }
+\cs_new_protected:Npn \tl_greverse:N #1
+ { \tl_gset:Nx #1 { \exp_args:No \tl_reverse:n { #1 } } }
+\cs_generate_variant:Nn \tl_reverse:N { c }
+\cs_generate_variant:Nn \tl_greverse:N { c }
+% \end{macrocode}
+% \end{macro}
+%
+% \subsection{The first token from a token list}
+%
+% \begin{macro}{\tl_head:N, \tl_head:n, \tl_head:V, \tl_head:v, \tl_head:f}
+% \begin{macro}{\@@_head_auxi:nw, \@@_head_auxii:n}
+% \begin{macro}{\tl_head:w}
+% \begin{macro}{\tl_tail:N, \tl_tail:n, \tl_tail:V, \tl_tail:v, \tl_tail:f}
+% Finding the head of a token list expandably always strips braces, which
+% is fine as this is consistent with for example mapping to a list. The
+% empty brace groups in \cs{tl_head:n} ensure that a blank argument gives an
+% empty result. The result is returned within the \tn{unexpanded} primitive.
+% The approach here is to use \cs{if_false:} to allow us to use |}| as
+% the closing delimiter: this is the only safe choice, as any other token
+% would not be able to parse it's own code. Using a marker, we can see if
+% what we are grabbing is exactly the marker, or there is anything else to
+% deal with. Is there is, there is a loop. If not, tidy up and leave the
+% item in the output stream. More detail in
+% \url{http://tex.stackexchange.com/a/70168}.
+% \begin{macrocode}
+\cs_new:Npn \tl_head:n #1
+ {
+ \__kernel_exp_not:w
+ \if_false: { \fi: \@@_head_auxi:nw #1 { } \q_stop }
+ }
+\cs_new:Npn \@@_head_auxi:nw #1#2 \q_stop
+ {
+ \exp_after:wN \@@_head_auxii:n \exp_after:wN {
+ \if_false: } \fi: {#1}
+ }
+\cs_new:Npn \@@_head_auxii:n #1
+ {
+ \exp_after:wN \if_meaning:w \exp_after:wN \q_nil
+ \__kernel_tl_to_str:w \exp_after:wN { \use_none:n #1 } \q_nil
+ \exp_after:wN \use_i:nn
+ \else:
+ \exp_after:wN \use_ii:nn
+ \fi:
+ {#1}
+ { \if_false: { \fi: \@@_head_auxi:nw #1 } }
+ }
+\cs_generate_variant:Nn \tl_head:n { V , v , f }
+\cs_new:Npn \tl_head:w #1#2 \q_stop {#1}
+\cs_new:Npn \tl_head:N { \exp_args:No \tl_head:n }
+% \end{macrocode}
+% To correctly leave the tail of a token list, it's important \emph{not} to
+% absorb any of the tail part as an argument. For example, the simple
+% definition
+% \begin{verbatim}
+% \cs_new:Npn \tl_tail:n #1 { \tl_tail:w #1 \q_stop }
+% \cs_new:Npn \tl_tail:w #1#2 \q_stop
+% \end{verbatim}
+% would give the wrong result for |\tl_tail:n { a { bc } }| (the braces would
+% be stripped). Thus the only safe way to proceed is to first check that
+% there is an item to grab (\emph{i.e.}~that the argument is not blank) and
+% assuming there is to dispose of the first item. As with \cs{tl_head:n},
+% the result is protected from further expansion by \tn{unexpanded}.
+% While we could optimise the test here, this would leave some tokens
+% \enquote{banned} in the input, which we do not have with this definition.
+% \begin{macrocode}
+\cs_new:Npn \tl_tail:n #1
+ {
+ \__kernel_exp_not:w
+ \tl_if_blank:nTF {#1}
+ { { } }
+ { \exp_after:wN { \use_none:n #1 } }
+ }
+\cs_generate_variant:Nn \tl_tail:n { V , v , f }
+\cs_new:Npn \tl_tail:N { \exp_args:No \tl_tail:n }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}[pTF]{\tl_if_head_eq_meaning:nN}
+% \begin{macro}[pTF]{\tl_if_head_eq_charcode:nN}
+% \begin{macro}[pTF]{\tl_if_head_eq_charcode:fN}
+% \begin{macro}[pTF]{\tl_if_head_eq_catcode:nN}
+% Accessing the first token of a token list is tricky in three cases:
+% when it has category code $1$ (begin-group token), when it is an
+% explicit space, with category code $10$ and character code $32$, or
+% when the token list is empty (obviously).
+%
+% Forgetting temporarily about this issue we would use the following
+% test in \cs{tl_if_head_eq_charcode:nN}. Here, \cs{tl_head:w} yields
+% the first token of the token list, then passed to \cs{exp_not:N}.
+% \begin{verbatim}
+% \if_charcode:w
+% \exp_after:wN \exp_not:N \tl_head:w #1 \q_nil \q_stop
+% \exp_not:N #2
+% \end{verbatim}
+% The two first special cases are detected by testing if the token
+% list starts with an \texttt{N}-type token (the extra |?| sends empty
+% token lists to the \texttt{true} branch of this test). In those
+% cases, the first token is a character, and since we only care about
+% its character code, we can use \cs{str_head:n} to access it (this
+% works even if it is a space character). An empty argument
+% results in \cs{tl_head:w} leaving two tokens: |?| which is taken in
+% the \cs{if_charcode:w} test, and \cs{use_none:nn}, which ensures
+% that \cs{prg_return_false:} is returned regardless of whether the
+% charcode test was \texttt{true} or \texttt{false}.
+% \begin{macrocode}
+\prg_new_conditional:Npnn \tl_if_head_eq_charcode:nN #1#2 { p , T , F , TF }
+ {
+ \if_charcode:w
+ \exp_not:N #2
+ \tl_if_head_is_N_type:nTF { #1 ? }
+ {
+ \exp_after:wN \exp_not:N
+ \tl_head:w #1 { ? \use_none:nn } \q_stop
+ }
+ { \str_head:n {#1} }
+ \prg_return_true:
+ \else:
+ \prg_return_false:
+ \fi:
+ }
+\prg_generate_conditional_variant:Nnn \tl_if_head_eq_charcode:nN
+ { f } { p , TF , T , F }
+% \end{macrocode}
+% For \cs{tl_if_head_eq_catcode:nN}, again we detect special cases
+% with a \cs{tl_if_head_is_N_type:n}. Then we need to test if the
+% first token is a begin-group token or an explicit space token, and
+% produce the relevant token, either \cs{c_group_begin_token} or
+% \cs{c_space_token}. Again, for an empty argument, a hack is used,
+% removing \cs{prg_return_true:} and \cs{else:} with \cs{use_none:nn}
+% in case the catcode test with the (arbitrarily chosen) |?| is
+% \texttt{true}.
+% \begin{macrocode}
+\prg_new_conditional:Npnn \tl_if_head_eq_catcode:nN #1 #2 { p , T , F , TF }
+ {
+ \if_catcode:w
+ \exp_not:N #2
+ \tl_if_head_is_N_type:nTF { #1 ? }
+ {
+ \exp_after:wN \exp_not:N
+ \tl_head:w #1 { ? \use_none:nn } \q_stop
+ }
+ {
+ \tl_if_head_is_group:nTF {#1}
+ { \c_group_begin_token }
+ { \c_space_token }
+ }
+ \prg_return_true:
+ \else:
+ \prg_return_false:
+ \fi:
+ }
+% \end{macrocode}
+% For \cs{tl_if_head_eq_meaning:nN}, again, detect special cases. In
+% the normal case, use \cs{tl_head:w}, with no \cs{exp_not:N} this
+% time, since \cs{if_meaning:w} causes no expansion. With an empty
+% argument, the test is \texttt{true}, and \cs{use_none:nnn} removes
+% |#2| and the usual \cs{prg_return_true:} and \cs{else:}.
+% In the special cases, we know that the first token is a character,
+% hence \cs{if_charcode:w} and \cs{if_catcode:w} together are enough.
+% We combine them in some order, hopefully faster than the reverse.
+% Tests are not nested because the arguments may contain unmatched
+% primitive conditionals.
+% \begin{macrocode}
+\prg_new_conditional:Npnn \tl_if_head_eq_meaning:nN #1#2 { p , T , F , TF }
+ {
+ \tl_if_head_is_N_type:nTF { #1 ? }
+ { \@@_if_head_eq_meaning_normal:nN }
+ { \@@_if_head_eq_meaning_special:nN }
+ {#1} #2
+ }
+\cs_new:Npn \@@_if_head_eq_meaning_normal:nN #1 #2
+ {
+ \exp_after:wN \if_meaning:w
+ \tl_head:w #1 { ?? \use_none:nnn } \q_stop #2
+ \prg_return_true:
+ \else:
+ \prg_return_false:
+ \fi:
+ }
+\cs_new:Npn \@@_if_head_eq_meaning_special:nN #1 #2
+ {
+ \if_charcode:w \str_head:n {#1} \exp_not:N #2
+ \exp_after:wN \use:n
+ \else:
+ \prg_return_false:
+ \exp_after:wN \use_none:n
+ \fi:
+ {
+ \if_catcode:w \exp_not:N #2
+ \tl_if_head_is_group:nTF {#1}
+ { \c_group_begin_token }
+ { \c_space_token }
+ \prg_return_true:
+ \else:
+ \prg_return_false:
+ \fi:
+ }
+ }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}[pTF]{\tl_if_head_is_N_type:n}
+% \begin{macro}[EXP]{\@@_if_head_is_N_type:w}
+% A token list can be empty, can start with an explicit space
+% character (catcode 10 and charcode 32), can start with a begin-group
+% token (catcode 1), or start with an \texttt{N}-type argument. In
+% the first two cases, the line involving \cs{@@_if_head_is_N_type:w}
+% produces~|^| (and otherwise nothing). In the third case
+% (begin-group token), the lines involving \cs{exp_after:wN} produce a
+% single closing brace. The category code test is thus true exactly
+% in the fourth case, which is what we want. One cannot optimize by
+% moving one of the |*| to the beginning: if |#1| contains primitive
+% conditionals, all of its occurrences must be dealt with before the
+% \cs{if_catcode:w} tries to skip the \texttt{true} branch of the
+% conditional.
+% \begin{macrocode}
+\prg_new_conditional:Npnn \tl_if_head_is_N_type:n #1 { p , T , F , TF }
+ {
+ \if_catcode:w
+ \if_false: { \fi: \@@_if_head_is_N_type:w ? #1 ~ }
+ \exp_after:wN \use_none:n
+ \exp_after:wN { \exp_after:wN { \token_to_str:N #1 ? } }
+ * *
+ \prg_return_true:
+ \else:
+ \prg_return_false:
+ \fi:
+ }
+\cs_new:Npn \@@_if_head_is_N_type:w #1 ~
+ {
+ \tl_if_empty:oTF { \use_none:n #1 } { ^ } { }
+ \exp_after:wN \use_none:n \exp_after:wN { \if_false: } \fi:
+ }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}[EXP,pTF]{\tl_if_head_is_group:n}
+% Pass the first token of |#1| through \cs{token_to_str:N}, then check
+% for the brace balance. The extra \texttt{?} caters for an empty
+% argument. This could be made faster, but we need all brace tricks
+% to happen in one step of expansion, keeping the token list brace
+% balanced at all times.
+% \begin{macrocode}
+\prg_new_conditional:Npnn \tl_if_head_is_group:n #1 { p , T , F , TF }
+ {
+ \if_catcode:w
+ \exp_after:wN \use_none:n
+ \exp_after:wN { \exp_after:wN { \token_to_str:N #1 ? } }
+ * *
+ \prg_return_false:
+ \else:
+ \prg_return_true:
+ \fi:
+ }
+% \end{macrocode}
+% \end{macro}
+%
+% \begin{macro}[EXP,pTF]{\tl_if_head_is_space:n}
+% \begin{macro}[EXP]{\@@_if_head_is_space:w}
+% The auxiliary's argument is all that is before the first explicit
+% space in |?#1?~|. If that is a single~|?| the test yields
+% \texttt{true}. Otherwise, that is more than one token, and the test
+% yields \texttt{false}. The work is done within braces (with an
+% |\if_false: { \fi: ... }| construction) both to hide potential
+% alignment tab characters from \TeX{} in a table, and to allow for
+% removing what remains of the token list after its first space. The
+% \cs{exp:w} and \cs{exp_end:} ensure that the result of a
+% single step of expansion directly yields a balanced token list (no
+% trailing closing brace).
+% \begin{macrocode}
+\prg_new_conditional:Npnn \tl_if_head_is_space:n #1 { p , T , F , TF }
+ {
+ \exp:w \if_false: { \fi:
+ \@@_if_head_is_space:w ? #1 ? ~ }
+ }
+\cs_new:Npn \@@_if_head_is_space:w #1 ~
+ {
+ \tl_if_empty:oTF { \use_none:n #1 }
+ { \exp_after:wN \exp_end: \exp_after:wN \prg_return_true: }
+ { \exp_after:wN \exp_end: \exp_after:wN \prg_return_false: }
+ \exp_after:wN \use_none:n \exp_after:wN { \if_false: } \fi:
+ }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \subsection{Using a single item}
+%
+% \begin{macro}{\tl_item:nn, \tl_item:Nn, \tl_item:cn}
+% \begin{macro}{\@@_item_aux:nn, \@@_item:nn}
+% The idea here is to find the offset of the item from the left, then use
+% a loop to grab the correct item. If the resulting offset is too large,
+% then \cs{quark_if_recursion_tail_stop:n} terminates the loop, and returns
+% nothing at all.
+% \begin{macrocode}
+\cs_new:Npn \tl_item:nn #1#2
+ {
+ \exp_args:Nf \@@_item:nn
+ { \exp_args:Nf \@@_item_aux:nn { \int_eval:n {#2} } {#1} }
+ #1
+ \q_recursion_tail
+ \prg_break_point:
+ }
+\cs_new:Npn \@@_item_aux:nn #1#2
+ {
+ \int_compare:nNnTF {#1} < 0
+ { \int_eval:n { \tl_count:n {#2} + 1 + #1 } }
+ {#1}
+ }
+\cs_new:Npn \@@_item:nn #1#2
+ {
+ \quark_if_recursion_tail_break:nN {#2} \prg_break:
+ \int_compare:nNnTF {#1} = 1
+ { \prg_break:n { \exp_not:n {#2} } }
+ { \exp_args:Nf \@@_item:nn { \int_eval:n { #1 - 1 } } }
+ }
+\cs_new:Npn \tl_item:Nn { \exp_args:No \tl_item:nn }
+\cs_generate_variant:Nn \tl_item:Nn { c }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}{\tl_rand_item:n, \tl_rand_item:N, \tl_rand_item:c}
+% Importantly \cs{tl_item:nn} only evaluates its argument once.
+% \begin{macrocode}
+\cs_new:Npn \tl_rand_item:n #1
+ {
+ \tl_if_blank:nF {#1}
+ { \tl_item:nn {#1} { \int_rand:nn { 1 } { \tl_count:n {#1} } } }
+ }
+\cs_new:Npn \tl_rand_item:N { \exp_args:No \tl_rand_item:n }
+\cs_generate_variant:Nn \tl_rand_item:N { c }
+% \end{macrocode}
+% \end{macro}
+%
+% \begin{macro}{\tl_range:Nnn, \tl_range:cnn, \tl_range:nnn}
+% \begin{macro}
+% {
+% \@@_range:Nnnn, \@@_range:nnnNn, \@@_range:nnNn, \@@_range_skip:w,
+% \@@_range:w, \@@_range_skip_spaces:n, \@@_range_collect:nn,
+% \@@_range_collect:ff, \@@_range_collect_space:nw,
+% \@@_range_collect_N:nN, \@@_range_collect_group:nN,
+% }
+% To avoid checking for the end of the token list at every step, start
+% by counting the number $l$ of items and \enquote{normalizing} the
+% bounds, namely clamping them to the interval $[0,l]$ and dealing
+% with negative indices. More precisely, \cs{@@_range_items:nnNn}
+% receives the number of items to skip at the beginning of the token
+% list, the index of the last item to keep, a function which is either
+% \cs{@@_range:w} or the token list itself. If nothing should be kept,
+% leave |{}|: this stops the \texttt{f}-expansion of \cs{tl_head:f} and that
+% function produces an empty result. Otherwise, repeatedly call
+% \cs{@@_range_skip:w} to delete |#1| items from the input stream (the
+% extra brace group avoids an off-by-one shift). For the braced
+% version \cs{@@_range_braced:w} sets up
+% \cs{@@_range_collect_braced:w} which stores items one by one in an
+% argument after the semicolon. Depending on the first token of the tail,
+% either just move it (if it is a space) or also decrement the number of
+% items left to find. Eventually, the result is a brace group followed by
+% the rest of the token list, and \cs{tl_head:f} cleans up and gives the
+% result in \cs{exp_not:n}.
+% \begin{macrocode}
+\cs_new:Npn \tl_range:Nnn { \exp_args:No \tl_range:nnn }
+\cs_generate_variant:Nn \tl_range:Nnn { c }
+\cs_new:Npn \tl_range:nnn { \@@_range:Nnnn \@@_range:w }
+\cs_new:Npn \@@_range:Nnnn #1#2#3#4
+ {
+ \tl_head:f
+ {
+ \exp_args:Nf \@@_range:nnnNn
+ { \tl_count:n {#2} } {#3} {#4} #1 {#2}
+ }
+ }
+\cs_new:Npn \@@_range:nnnNn #1#2#3
+ {
+ \exp_args:Nff \@@_range:nnNn
+ {
+ \exp_args:Nf \@@_range_normalize:nn
+ { \int_eval:n { #2 - 1 } } {#1}
+ }
+ {
+ \exp_args:Nf \@@_range_normalize:nn
+ { \int_eval:n {#3} } {#1}
+ }
+ }
+\cs_new:Npn \@@_range:nnNn #1#2#3#4
+ {
+ \if_int_compare:w #2 > #1 \exp_stop_f: \else:
+ \exp_after:wN { \exp_after:wN }
+ \fi:
+ \exp_after:wN #3
+ \int_value:w \int_eval:n { #2 - #1 } \exp_after:wN ;
+ \exp_after:wN { \exp:w \@@_range_skip:w #1 ; { } #4 }
+ }
+\cs_new:Npn \@@_range_skip:w #1 ; #2
+ {
+ \if_int_compare:w #1 > 0 \exp_stop_f:
+ \exp_after:wN \@@_range_skip:w
+ \int_value:w \int_eval:n { #1 - 1 } \exp_after:wN ;
+ \else:
+ \exp_after:wN \exp_end:
+ \fi:
+ }
+\cs_new:Npn \@@_range:w #1 ; #2
+ {
+ \exp_args:Nf \@@_range_collect:nn
+ { \@@_range_skip_spaces:n {#2} } {#1}
+ }
+\cs_new:Npn \@@_range_skip_spaces:n #1
+ {
+ \tl_if_head_is_space:nTF {#1}
+ { \exp_args:Nf \@@_range_skip_spaces:n {#1} }
+ { { } #1 }
+ }
+\cs_new:Npn \@@_range_collect:nn #1#2
+ {
+ \int_compare:nNnTF {#2} = 0
+ {#1}
+ {
+ \exp_args:No \tl_if_head_is_space:nTF { \use_none:n #1 }
+ {
+ \exp_args:Nf \@@_range_collect:nn
+ { \@@_range_collect_space:nw #1 }
+ {#2}
+ }
+ {
+ \@@_range_collect:ff
+ {
+ \exp_args:No \tl_if_head_is_N_type:nTF { \use_none:n #1 }
+ { \@@_range_collect_N:nN }
+ { \@@_range_collect_group:nn }
+ #1
+ }
+ { \int_eval:n { #2 - 1 } }
+ }
+ }
+ }
+\cs_new:Npn \@@_range_collect_space:nw #1 ~ { { #1 ~ } }
+\cs_new:Npn \@@_range_collect_N:nN #1#2 { { #1 #2 } }
+\cs_new:Npn \@@_range_collect_group:nn #1#2 { { #1 {#2} } }
+\cs_generate_variant:Nn \@@_range_collect:nn { ff }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}[EXP]{\@@_range_normalize:nn}
+% This function converts an \meta{index} argument into an explicit
+% position in the token list (a result of $0$ denoting \enquote{out of
+% bounds}). Expects two explicit integer arguments: the \meta{index}
+% |#1| and the string count~|#2|. If |#1| is negative, replace it by
+% $|#1| + |#2| + 1$, then limit to the range $[0, |#2|]$.
+% \begin{macrocode}
+\cs_new:Npn \@@_range_normalize:nn #1#2
+ {
+ \int_eval:n
+ {
+ \if_int_compare:w #1 < 0 \exp_stop_f:
+ \if_int_compare:w #1 < -#2 \exp_stop_f:
+ 0
+ \else:
+ #1 + #2 + 1
+ \fi:
+ \else:
+ \if_int_compare:w #1 < #2 \exp_stop_f:
+ #1
+ \else:
+ #2
+ \fi:
+ \fi:
+ }
+ }
+% \end{macrocode}
+% \end{macro}
+%
+% \subsection{Viewing token lists}
+%
+% \begin{macro}{\tl_show:N, \tl_show:c, \tl_log:N, \tl_log:c, \@@_show:NN}
+% Showing token list variables is done after checking that the
+% variable is defined (see \cs{__kernel_register_show:N}).
+% \begin{macrocode}
+\cs_new_protected:Npn \tl_show:N { \@@_show:NN \tl_show:n }
+\cs_generate_variant:Nn \tl_show:N { c }
+\cs_new_protected:Npn \tl_log:N { \@@_show:NN \tl_log:n }
+\cs_generate_variant:Nn \tl_log:N { c }
+\cs_new_protected:Npn \@@_show:NN #1#2
+ {
+ \__kernel_chk_defined:NT #2
+ { \exp_args:Nx #1 { \token_to_str:N #2 = \exp_not:o {#2} } }
+ }
+% \end{macrocode}
+% \end{macro}
+%
+% \begin{macro}{\tl_show:n, \@@_show:n}
+% \begin{macro}[EXP]{\@@_show:w}
+% Many |show| functions are based on \cs{tl_show:n}.
+% The argument of \cs{tl_show:n} is line-wrapped using
+% \cs{iow_wrap:nnnN} but with a leading |>~| and trailing period, both
+% removed before passing the wrapped text to the \tn{showtokens}
+% primitive. This primitive shows the result with a leading |>~| and
+% trailing period.
+%
+% The token list \cs{l_@@_internal_a_tl} containing the result
+% of all these manipulations is displayed to the terminal using
+% \cs{tex_showtokens:D} and an odd \cs{exp_after:wN} which expand the
+% closing brace to improve the output slightly. The calls to
+% \cs{__kernel_iow_with:Nnn} ensure that the \tn{newlinechar} is set to~$10$
+% so that the \cs{iow_newline:} inserted by the line-wrapping code
+% are correctly recognized by \TeX{}, and that \tn{errorcontextlines}
+% is $-1$ to avoid printing irrelevant context.
+% \begin{macrocode}
+\cs_new_protected:Npn \tl_show:n #1
+ { \iow_wrap:nnnN { >~ \tl_to_str:n {#1} . } { } { } \@@_show:n }
+\cs_new_protected:Npn \@@_show:n #1
+ {
+ \tl_set:Nf \l_@@_internal_a_tl { \@@_show:w #1 \q_stop }
+ \__kernel_iow_with:Nnn \tex_newlinechar:D { 10 }
+ {
+ \__kernel_iow_with:Nnn \tex_errorcontextlines:D { -1 }
+ {
+ \tex_showtokens:D \exp_after:wN \exp_after:wN \exp_after:wN
+ { \exp_after:wN \l_@@_internal_a_tl }
+ }
+ }
+ }
+\cs_new:Npn \@@_show:w #1 > #2 . \q_stop {#2}
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}{\tl_log:n}
+% Logging is much easier, simply line-wrap. The |>~| and trailing
+% period is there to match the output of \cs{tl_show:n}.
+% \begin{macrocode}
+\cs_new_protected:Npn \tl_log:n #1
+ { \iow_wrap:nnnN { > ~ \tl_to_str:n {#1} . } { } { } \iow_log:n }
+% \end{macrocode}
+% \end{macro}
+%
+% \subsection{Scratch token lists}
+%
+% \begin{variable}{\g_tmpa_tl, \g_tmpb_tl}
+% Global temporary token list variables.
+% They are supposed to be set and used immediately,
+% with no delay between the definition and the use because you
+% can't count on other macros not to redefine them from under you.
+% \begin{macrocode}
+\tl_new:N \g_tmpa_tl
+\tl_new:N \g_tmpb_tl
+% \end{macrocode}
+% \end{variable}
+%
+% \begin{variable}{\l_tmpa_tl, \l_tmpb_tl}
+% These are local temporary token list variables. Be sure not to assume
+% that the value you put into them will survive for
+% long---see discussion above.
+% \begin{macrocode}
+\tl_new:N \l_tmpa_tl
+\tl_new:N \l_tmpb_tl
+% \end{macrocode}
+% \end{variable}
+%
+% \begin{macrocode}
+%</initex|package>
+% \end{macrocode}
+%
+% \end{implementation}
+%
+% \PrintIndex