% \iffalse meta-comment
%
%% File: l3box.dtx Copyright (C) 2005-2011 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
%%
%%    http://www.latex-project.org/lppl.txt
%%
%% This file is part of the "expl3 bundle" (The Work in LPPL)
%% and all files in that bundle must be distributed together.
%%
%% The released version of this bundle is available from CTAN.
%%
%% -----------------------------------------------------------------------
%%
%% The development version of the bundle can be found at
%%
%%    http://www.latex-project.org/svnroot/experimental/trunk/
%%
%% for those people who are interested.
%%
%%%%%%%%%%%
%% NOTE: %%
%%%%%%%%%%%
%%
%%   Snapshots taken from the repository represent work in progress and may
%%   not work or may contain conflicting material!  We therefore ask
%%   people _not_ to put them into distributions, archives, etc. without
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%
%<*driver|package>
\RequirePackage{l3names}
\GetIdInfo$Id: l3box.dtx 2478 2011-06-19 21:34:23Z joseph $
  {L3 Experimental boxes}
%</driver|package>
%<*driver>
\documentclass[full]{l3doc}
\begin{document}
  \DocInput{\jobname.dtx}
\end{document}
%</driver>
% \fi
%
% \title{^^A
%   The \pkg{l3box} package\\ Boxes^^A
%   \thanks{This file describes v\ExplFileVersion,
%      last revised \ExplFileDate.}^^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 \ExplFileDate}
%
% \maketitle
%
% \begin{documentation}
%
% There are three kinds of box operations: horizontal mode denoted
% with prefix |\hbox_|, vertical mode with prefix |\vbox_|, and the
% generic operations working in both modes with prefix |\box_|.
%
% \section{Creating and initialising boxes}
%
% \begin{function}{\box_new:N, \box_new:c}
%   \begin{syntax}
%     \cs{box_new:N} \meta{box}
%   \end{syntax}
%   Creates a new \meta{box} or raises an error if the name is
%   already taken. The declaration is global. The \meta{box} will
%   initially be void.
% \end{function}
%
% \begin{function}{\box_clear:N, \box_clear:c}
%   \begin{syntax}
%     \cs{box_clear:N} \meta{box}
%   \end{syntax}
%   Clears the content of the \meta{box} by setting the box equal to
%   \cs{c_void_box} within the current \TeX{} group level.
% \end{function}
%
% \begin{function}{\box_gclear:N, \box_gclear:c}
%   \begin{syntax}
%     \cs{box_gclear:N} \meta{box}
%   \end{syntax}
%   Clears the content of the \meta{box} by setting the box equal to
%   \cs{c_void_box} globally.
% \end{function}
%
% \begin{function}{\box_clear_new:N, \box_clear_new:c}
%   \begin{syntax}
%     \cs{box_clear_new:N} \meta{box}
%   \end{syntax}
%   If the \meta{box} is not defined, globally creates it. If the \meta{box}
%   is defined,
%   clears the content of the \meta{box} by setting the box equal to
%   \cs{c_void_box} within the current \TeX{} group level.
% \end{function}
%
% \begin{function}{\box_gclear_new:N, \box_gclear_new:c}
%   \begin{syntax}
%     \cs{box_gclear_new:N} \meta{box}
%   \end{syntax}
%   If the \meta{box} is not defined, globally creates it. If the \meta{box}
%   is defined,
%   clears the content of the \meta{box} by setting the box equal to
%   \cs{c_void_box} globally.
% \end{function}
%
% \begin{function}
%   {\box_set_eq:NN, \box_set_eq:cN, \box_set_eq:Nc, \box_set_eq:cc}
%   \begin{syntax}
%     \cs{box_set_eq:NN} \meta{box1} \meta{box2}
%   \end{syntax}
%   Sets the content of \meta{box1} equal to that of \meta{box2}. This
%   assignment is restricted to the current \TeX{} group level.
% \end{function}
%
% \begin{function}
%   {\box_gset_eq:NN, \box_gset_eq:cN, \box_gset_eq:Nc, \box_gset_eq:cc}
%   \begin{syntax}
%     \cs{box_gset_eq:NN} \meta{box1} \meta{box2}
%   \end{syntax}
%   Sets the content of \meta{box1} equal to that of \meta{box2} globally.
% \end{function}
%
% \begin{function}
%   {
%     \box_set_eq_clear:NN, \box_set_eq_clear:cN,
%     \box_set_eq_clear:Nc, \box_set_eq_clear:cc
%   }
%   \begin{syntax}
%     \cs{box_set_eq_clear:NN} \meta{box1} \meta{box2}
%   \end{syntax}
%   Sets the content of \meta{box1} within the current \TeX{} group
%   equal to that of \meta{box2}, then clears \meta{box2} globally.
% \end{function}
%
% \begin{function}
%   {
%     \box_gset_eq_clear:NN, \box_gset_eq_clear:cN,
%     \box_gset_eq_clear:Nc, \box_gset_eq_clear:cc
%   }
%   \begin{syntax}
%     \cs{box_gset_eq_clear:NN} \meta{box1} \meta{box2}
%   \end{syntax}
%   Sets the content of \meta{box1} equal to that of \meta{box2}, then
%   clears \meta{box2}. These assignments are global.
% \end{function}
%
% \section{Using boxes}
%
% \begin{function}{\box_use:N, \box_use:c}
%   \begin{syntax}
%     \cs{box_use:N} \meta{box}
%   \end{syntax}
%   Inserts the current content of the \meta{box} onto the current
%   list for typesetting.
%   \begin{texnote}
%     This is the \TeX{} primitive \cs{copy}.
%   \end{texnote}
% \end{function}
%
% \begin{function}{\box_use_clear:N, \box_use_clear:c}
%   \begin{syntax}
%     \cs{box_use_clear:N} \meta{box}
%   \end{syntax}
%   Inserts the current content of the \meta{box} onto the current
%   list for typesetting, then globally clears the content of the
%   \meta{box}.
%   \begin{texnote}
%     This is the \TeX{} primitive \cs{box}.
%   \end{texnote}
% \end{function}
%
% \begin{function}{\box_move_right:nn, \box_move_left:nn}
%   \begin{syntax}
%     \cs{box_move_right:nn} \Arg{dimexpr} \Arg{box function}
%   \end{syntax}
%   This function operates in vertical mode, and inserts the
%   material specified by the \meta{box function}
%   such that its reference point is displaced horizontally by the given
%   \meta{dimexpr} from the reference point for typesetting, to the right
%   or left as appropriate. The \meta{box function} should be
%   a box opeeration such as |\box_use:N \<box>| or a \enquote{raw}
%   box specification such as |\vbox:n { xyz }|.
% \end{function}
%
% \begin{function}{\box_move_up:nn, \box_move_down:nn}
%   \begin{syntax}
%     \cs{box_move_up:nn} \Arg{dimexpr} \Arg{box function}
%   \end{syntax}
%   This function operates in horizontal mode, and inserts the
%   material specified by the \meta{box function}
%   such that its reference point is displaced vertical by the given
%   \meta{dimexpr} from the reference point for typesetting, up
%   or down as appropriate. The \meta{box function} should be
%   a box opeeration such as |\box_use:N \<box>| or a \enquote{raw}
%   box specification such as |\vbox:n { xyz }|.
% \end{function}
%
% \section{Measuring and setting box dimensions}
%
% \begin{function}{\box_dp:N, \box_dp:c}
%   \begin{syntax}
%     \cs{box_dp:N} \meta{box}
%   \end{syntax}
%   Calculates the depth (below the baseline) of the \meta{box}
%   and leaves this in the input stream. The output of this function
%   is suitable for use in a \meta{dimension expression} for
%   calculations.
%   \begin{texnote}
%     This is the \TeX{} primitive \cs{dp}.
%   \end{texnote}
% \end{function}
%
% \begin{function}{\box_ht:N, \box_ht:c}
%   \begin{syntax}
%     \cs{box_ht:N} \meta{box}
%   \end{syntax}
%   Calculates the height (above the baseline) of the \meta{box}
%   and leaves this in the input stream. The output of this function
%   is suitable for use in a \meta{dimension expression} for
%   calculations.
%   \begin{texnote}
%     This is the \TeX{} primitive \cs{ht}.
%   \end{texnote}
% \end{function}
%
% \begin{function}{\box_wd:N, \box_wd:c}
%   \begin{syntax}
%     \cs{box_wd:N} \meta{box}
%   \end{syntax}
%   Calculates the width of the \meta{box} and leaves this in the
%   input stream. The output of this function is suitable for use in a
%   \meta{dimension expression} for calculations.
%   \begin{texnote}
%     This is the \TeX{} primitive \cs{wd}.
%   \end{texnote}
% \end{function}
%
% \begin{function}{\box_set_dp:Nn, \box_set_dp:cn}
%   \begin{syntax}
%     \cs{box_set_dp:Nn} \meta{box} \Arg{dimension expression}
%   \end{syntax}
%   Set the depth (below the baseline) of the \meta{box} to the value of
%   the \Arg{dimension expression}. This is a global assignment.
% \end{function}
%
% \begin{function}{\box_set_ht:Nn, \box_set_ht:cn}
%   \begin{syntax}
%     \cs{box_set_ht:Nn} \meta{box} \Arg{dimension expression}
%   \end{syntax}
%   Set the height (above the baseline) of the \meta{box} to the value of
%   the \Arg{dimension expression}. This is a global assignment.
% \end{function}
%
% \begin{function}{\box_set_wd:Nn, \box_set_wd:cn}
%   \begin{syntax}
%     \cs{box_set_wd:Nn} \meta{box} \Arg{dimension expression}
%   \end{syntax}
%   Set the width of the \meta{box} to the value of the
%   \Arg{dimension expression}. This is a global assignment.
% \end{function}
%
% \section{Box conditionals}
%
% \begin{function}[EXP,pTF]{\box_if_empty:N, \box_if_empty:c}
%   \begin{syntax}
%     \cs{box_if_empty_p:N} \meta{box}
%     \cs{box_if_empty:NTF} \meta{box} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if \meta{box} is a empty (equal to \cs{c_empty_box}).
%   The branching versions then leave either
%   \meta{true code} or \meta{false code} in the input stream, as
%   appropriate to the truth of the test and the variant of the function
%   chosen. The logical truth of the test is left in the input stream by
%   the predicate version.
% \end{function}
%
% \begin{function}[EXP,pTF]{\box_if_horizontal:N, \box_if_horizontal:c}
%   \begin{syntax}
%     \cs{box_if_horizontal_p:N} \meta{box}
%     \cs{box_if_horizontal:NTF} \meta{box} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if \meta{box} is a horizontal box. The branching
%   versions then leave either \meta{true code} or \meta{false code}
%   in the input stream, as appropriate to the truth of the test and
%   the variant of the function chosen. The logical truth of the test
%   is left in the input stream by the predicate version.
% \end{function}
%
% \begin{function}[EXP,pTF]{\box_if_vertical:N, \box_if_vertical:c}
%   \begin{syntax}
%     \cs{box_if_vertical_p:N} \meta{box}
%     \cs{box_if_vertical:NTF} \meta{box} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if \meta{box} is a vertical box. The branching
%   versions then leave either \meta{true code} or \meta{false code}
%   in the input stream, as appropriate to the truth of the test and
%   the variant of the function chosen. The logical truth of the test
%   is left in the input stream by the predicate version.
% \end{function}
%
% \section{The last box inserted}
%
% \begin{variable}{\l_last_box}
%   This is a box containing the last item added to the current partial
%   list, except in the case of the main vertical list (main galley), in
%   which case this box is always void. Notice that although this is
%   not a constant, it is \emph{not} settable by the programmer but is
%   instead varied by \TeX{}.
%   \begin{texnote}
%     This is the \TeX{} primitive \cs{lastbox} renamed.
%   \end{texnote}
% \end{variable}
%
% \section{Constant boxes}
%
%  \begin{variable}{\c_empty_box}
%    This is a permanently empty box, which is neither set as horizontal
%    nor vertical.
%  \end{variable}
%
%  \section{Scratch boxes}
%
% \begin{variable}{\l_tmpa_tl, \l_tmpb_tl}
%   Scratch boxes 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}
%
% \section{Viewing box contents}
%
% \begin{function}{\box_show:N, \box_show:c}
%   \begin{syntax}
%      \cs{box_show:N} \meta{box}
%   \end{syntax}
%   Writes the contents of \meta{box} to the log file.
%   \begin{texnote}
%     This is the \TeX{} primitive \cs{showbox}.
%   \end{texnote}
% \end{function}
%
% \section{Horizontal mode boxes}
%
% \begin{function}{\hbox:n}
%   \begin{syntax}
%     \cs{hbox:n} \Arg{contents}
%   \end{syntax}
%   Typesets the \meta{contents} into a horizontal box of natural
%   width and then includes this box in the current list for typesetting.
%   \begin{texnote}
%     This is the \TeX{} primitive \cs{hbox}.
%   \end{texnote}
% \end{function}
%
% \begin{function}{\hbox_to_wd:nn}
%   \begin{syntax}
%     \cs{hbox_to_wd:nn} \Arg{dimexpr} \Arg{contents}
%   \end{syntax}
%   Typesets the \meta{contents} into a horizontal box of width
%   \meta{dimexpr} and then includes this box in the current list for
%   typesetting.
% \end{function}
%
% \begin{function}{\hbox_to_zero:n}
%   \begin{syntax}
%     \cs{hbox_to_zero:n} \Arg{contents}
%   \end{syntax}
%   Typesets the \meta{contents} into a horizontal box of zero width
%   and then includes this box in the current list for typesetting.
% \end{function}
%
% \begin{function}{\hbox_set:Nn, \hbox_set:cn}
%   \begin{syntax}
%     \cs{hbox_set:Nn} \meta{box} \Arg{contents}
%   \end{syntax}
%   Typesets the \meta{contents} at natural width and then stores the
%   result inside the \meta{box}. The assignment is local.
% \end{function}
%
% \begin{function}{\hbox_gset:Nn, \hbox_gset:cn}
%   \begin{syntax}
%     \cs{hbox_gset:Nn} \meta{box} \Arg{contents}
%   \end{syntax}
%   Typesets the \meta{contents} at natural width and then stores the
%   result inside the \meta{box}. The assignment is global.
% \end{function}
%
% \begin{function}{\hbox_set_to_wd:Nnn, \hbox_set_to_wd:cnn}
%   \begin{syntax}
%     \cs{hbox_set_to_wd:Nnn} \meta{box} \Arg{dimexpr} \Arg{contents}
%   \end{syntax}
%   Typesets the \meta{contents} to the width given by the \meta{dimexpr}
%   and then stores the result inside the \meta{box}. The assignment is
%   local.
% \end{function}
%
% \begin{function}{\hbox_gset_to_wd:Nnn, \hbox_gset_to_wd:cnn}
%   \begin{syntax}
%     \cs{hbox_gset_to_wd:Nnn} \meta{box} \Arg{dimexpr} \Arg{contents}
%   \end{syntax}
%   Typesets the \meta{contents} to the width given by the \meta{dimexpr}
%   and then stores the result inside the \meta{box}. The assignment is
%   global.
% \end{function}
%
% \begin{function}{\hbox_overlap_right:n}
%   \begin{syntax}
%     \cs{hbox_overlap_right:n} \Arg{contents}
%   \end{syntax}
%   Typesets the \meta{contents} into a horizontal box of zero width
%   such that material will protrude to the right of the insertion
%   point.
% \end{function}
%
% \begin{function}{\hbox_overlap_left:n}
%   \begin{syntax}
%     \cs{hbox_overlap_left:n} \Arg{contents}
%   \end{syntax}
%   Typesets the \meta{contents} into a horizontal box of zero width
%   such that material will protrude to the left of the insertion
%   point.
% \end{function}
%
% \begin{function}
%   {
%     \hbox_set_inline_begin:N, \hbox_set_inline_begin:c,
%     \hbox_set_inline_end:
%   }
%   \begin{syntax}
%     \cs{hbox_set_inline_begin:N} \meta{box} \meta{contents}
%     ~~\cs{hbox_set_inline_end:}
%   \end{syntax}
%   Typesets the \meta{contents} at natural width and then stores the
%   result inside the \meta{box}. The assignment is local. In contrast
%   to \cs{hbox_set:Nn} this function does not absorb the argument
%   when finding the \meta{content}, and so can be used in circumstances
%   where the \meta{content} may not be a simple argument.
% \end{function}
%
% \begin{function}
%   {
%     \hbox_gset_inline_begin:N, \hbox_gset_inline_begin:c,
%     \hbox_gset_inline_end:
%   }
%   \begin{syntax}
%     \cs{hbox_gset_inline_begin:N} \meta{box} \meta{contents}
%     ~~\cs{hbox_gset_inline_end:}
%   \end{syntax}
%   Typesets the \meta{contents} at natural width and then stores the
%   result inside the \meta{box}. The assignment is global. In contrast
%   to \cs{hbox_set:Nn} this function does not absorb the argument
%   when finding the \meta{content}, and so can be used in circumstances
%   where the \meta{content} may not be a simple argument.
% \end{function}
%
% \begin{function}{\hbox_unpack:N, \hbox_unpack:c}
%   \begin{syntax}
%     \cs{hbox_unpack:N} \meta{box}
%   \end{syntax}
%   Unpacks the content of the horizontal \meta{box}, retaining any stretching
%   or shrinking applied when the \meta{box} was set.
%   \begin{texnote}
%     This is the \TeX{} primitive \cs{unhcopy}.
%   \end{texnote}
% \end{function}
%
% \begin{function}{\hbox_unpack_clear:N, \hbox_unpack_clear:c}
%   \begin{syntax}
%     \cs{hbox_unpack_clear:N} \meta{box}
%   \end{syntax}
%   Unpacks the content of the horizontal \meta{box}, retaining any stretching
%   or shrinking applied when the \meta{box} was set. The \meta{box} is
%   then cleared globally.
%   \begin{texnote}
%     This is the \TeX{} primitive \cs{unhbox}.
%   \end{texnote}
% \end{function}
%
% \section{Vertical mode boxes}
%
% Vertical boxes inherit their baseline from their contents. The
% standard case is that the baseline of the box is at the same position
% as that of the last item added to the box. This means that the box
% will have no depth unless the last item added to it had depth. As a
% result most vertical boxes have a large height value and small or
% zero depth. The exception are |_top| boxes, where the reference point
% is that of the first item added. These tend to have a large depth and
% small height, although the later will typically be non-zero.
%
% \begin{function}{\vbox:n}
%   \begin{syntax}
%     \cs{vbox:n} \Arg{contents}
%   \end{syntax}
%   Typesets the \meta{contents} into a vertical box of natural height
%   and includes this box in the current list for typesetting.
%   \begin{texnote}
%     This is the \TeX{} primitive \cs{vbox}.
%   \end{texnote}
% \end{function}
%
% \begin{function}{\vbox_top:n}
%   \begin{syntax}
%     \cs{vbox_top:n} \Arg{contents}
%   \end{syntax}
%   Typesets the \meta{contents} into a vertical box of natural height
%   and includes this box in the current list for typesetting. The
%   baseline of the box will tbe equal to that of the \emph{first}
%   item added to the box.
%   \begin{texnote}
%     This is the \TeX{} primitive \cs{vtop}.
%   \end{texnote}
% \end{function}
%
% \begin{function}{\vbox_to_ht:nn}
%   \begin{syntax}
%     \cs{vbox_to_ht:n} \Arg{contents} \Arg{dimexpr}
%   \end{syntax}
%   Typesets the \meta{contents} into a vertical box of height
%   \meta{dimexpr} and then includes this box in the current list for
%   typesetting.
% \end{function}
%
% \begin{function}{\vbox_to_zero:n}
%   \begin{syntax}
%     \cs{vbox_to_zero:n} \Arg{contents}
%   \end{syntax}
%   Typesets the \meta{contents} into a vertical box of zero height
%   and then includes this box in the current list for typesetting.
% \end{function}
%
%  \begin{function}{\vbox_set:Nn, \vbox_set:cn}
%   \begin{syntax}
%     \cs{vbox_set:Nn} \meta{box} \Arg{contents}
%   \end{syntax}
%   Typesets the \meta{contents} at natural height and then stores the
%   result inside the \meta{box}. The assignment is local.
% \end{function}
%
%  \begin{function}{\vbox_gset:Nn, \vbox_gset:cn}
%   \begin{syntax}
%     \cs{vbox_gset:Nn} \meta{box} \Arg{contents}
%   \end{syntax}
%   Typesets the \meta{contents} at natural height and then stores the
%   result inside the \meta{box}. The assignment is global.
% \end{function}
%
% \begin{function}{\vbox_set_top:Nn, \vbox_set_top:cn}
%   \begin{syntax}
%     \cs{vbox_set_top:Nn} \meta{box} \Arg{contents}
%   \end{syntax}
%   Typesets the \meta{contents} at natural height and then stores the
%   result inside the \meta{box}. The baseline of the box will tbe equal
%   to that of the \emph{first} item added to the box. The assignment is
%   local.
% \end{function}
%
% \begin{function}{\vbox_gset_top:Nn, \vbox_gset_top:cn}
%   \begin{syntax}
%     \cs{vbox_gset_top:Nn} \meta{box} \Arg{contents}
%   \end{syntax}
%   Typesets the \meta{contents} at natural height and then stores the
%   result inside the \meta{box}. The baseline of the box will tbe equal
%   to that of the \emph{first} item added to the box. The assignment is
%   global.
% \end{function}
%
% \begin{function}{\vbox_set_to_ht:Nnn, \vbox_set_to_ht:cnn}
%   \begin{syntax}
%     \cs{vbox_set_to_ht:Nnn} \meta{box} \Arg{dimexpr} \Arg{contents}
%   \end{syntax}
%   Typesets the \meta{contents} to the height given by the
%   \meta{dimexpr} and then stores the result inside the \meta{box}.
%   The assignment is local.
% \end{function}
%
% \begin{function}{\vbox_gset_to_ht:Nnn, \vbox_gset_to_ht:cnn}
%   \begin{syntax}
%     \cs{vbox_gset_to_ht:Nnn} \meta{box} \Arg{dimexpr} \Arg{contents}
%   \end{syntax}
%   Typesets the \meta{contents} to the height given by the
%   \meta{dimexpr} and then stores the result inside the \meta{box}.
%   The assignment is global.
% \end{function}
%
% \begin{function}
%  {
%    \vbox_set_inline_begin:N, \vbox_set_inline_begin:c,
%    \vbox_set_inline_end:
%  }
%    \begin{syntax}
%      \cs{vbox_set_inline_begin:N} \meta{box} \meta{contents}
%      ~~\cs{vbox_set_inline_end:}
%    \end{syntax}
%    Typesets the \meta{contents} at natural height and then stores the
%    result inside the \meta{box}. The assignment is local. In contrast
%    to \cs{vbox_set:Nn} this function does not absorb the argument
%    when finding the \meta{content}, and so can be used in circumstances
%    where the \meta{content} may not be a simple argument.
%  \end{function}
%
% \begin{function}
%  {
%    \vbox_gset_inline_begin:N, \vbox_gset_inline_begin:c,
%    \vbox_gset_inline_end:
%  }
%    \begin{syntax}
%      \cs{vbox_gset_inline_begin:N} \meta{box} \meta{contents}
%      ~~\cs{vbox_set_inline_end:}
%    \end{syntax}
%    Typesets the \meta{contents} at natural height and then stores the
%    result inside the \meta{box}. The assignment is global. In contrast
%    to \cs{vbox_set:Nn} this function does not absorb the argument
%    when finding the \meta{content}, and so can be used in circumstances
%    where the \meta{content} may not be a simple argument.
%  \end{function}
%
% \begin{function}{\vbox_set_split_to_ht:NNn}
%   \begin{syntax}
%      \cs{vbox_set_split_to_ht:NNn} \meta{box1} \meta{box2} \Arg{dimexpr}
%   \end{syntax}
%   Sets \meta{box1} to contain material to the height given by the
%   \meta{dimexpr} by removing content from the top of \meta{box2}
%   (which must be a vertical box).
%   \begin{texnote}
%     This is the \TeX{} primitive \cs{vsplit}.
%   \end{texnote}
% \end{function}
%
% \begin{function}{\vbox_unpack:N, \vbox_unpack:c}
%   \begin{syntax}
%     \cs{vbox_unpack:N} \meta{box}
%   \end{syntax}
%   Unpacks the content of the vertical \meta{box}, retaining any stretching
%   or shrinking applied when the \meta{box} was set.
%   \begin{texnote}
%     This is the \TeX{} primitive \cs{unvcopy}.
%   \end{texnote}
% \end{function}
%
% \begin{function}{\vbox_unpack_clear:N, \vbox_unpack_clear:c}
%   \begin{syntax}
%     \cs{vbox_unpack:N} \meta{box}
%   \end{syntax}
%   Unpacks the content of the vertical \meta{box}, retaining any stretching
%   or shrinking applied when the \meta{box} was set. The \meta{box}
%   is then cleared globally.
%   \begin{texnote}
%     This is the \TeX{} primitive \cs{unvbox}.
%   \end{texnote}
% \end{function}
%
% \section{Primitive box conditionals}
%
% \begin{function}[EXP]{\if_hbox:N}
%   \begin{syntax}
%     \cs{if_hbox:N} \meta{box}
%     ~~\meta{true code}
%     \cs{else:}
%     ~~\meta{false code}
%     \cs{fi:}
%   \end{syntax}
%   Tests is \meta{box} is a horizontal box.
%   \begin{texnote}
%     This is the \TeX{} primitive \cs{ifhbox}.
%   \end{texnote}
% \end{function}
%
% \begin{function}[EXP]{\if_vbox:N}
%   \begin{syntax}
%     \cs{if_vbox:N} \meta{box}
%     ~~\meta{true code}
%     \cs{else:}
%     ~~\meta{false code}
%     \cs{fi:}
%   \end{syntax}
%   Tests is \meta{box} is a vertical box.
%   \begin{texnote}
%     This is the \TeX{} primitive \cs{ifvbox}.
%   \end{texnote}
% \end{function}
%
% \begin{function}[EXP]{\if_box_empty:N}
%   \begin{syntax}
%     \cs{if_box_empty:N} \meta{box}
%     ~~\meta{true code}
%     \cs{else:}
%     ~~\meta{false code}
%     \cs{fi:}
%   \end{syntax}
%   Tests is \meta{box} is an empty (void) box.
%   \begin{texnote}
%     This is the \TeX{} primitive \cs{ifvoid}.
%   \end{texnote}
% \end{function}
%
% \end{documentation}
%
% \begin{implementation}
%
% \section{\pkg{l3box} implementation}
%
%    \begin{macrocode}
%<*initex|package>
%    \end{macrocode}
%
%    \begin{macrocode}
%<*package>
\ProvidesExplPackage
  {\ExplFileName}{\ExplFileDate}{\ExplFileVersion}{\ExplFileDescription}
\package_check_loaded_expl:
%<*initex|package>
%    \end{macrocode}
%
%  The code in this module is very straight forward so I'm not going to
%  comment it very extensively.
%
% \subsection{Creating and initialising boxes}
%
% \TestFiles{m3box001.lvt}
%
% \begin{macro}{\box_new:N,\box_new:c}
%   Defining a new \meta{box} register: remember that box $255$ is not
%   generally available.
%    \begin{macrocode}
%<*initex>
\alloc_new:nnnN { box } \c_zero \c_max_register_int \tex_mathchardef:D
\seq_put_right:Nn \g_box_allocation_seq {255}
%</initex>
%<*package>
\cs_new_protected:Npn \box_new:N #1
  {
    \chk_if_free_cs:N #1
    \newbox #1
  }
%</package>
\cs_generate_variant:Nn \box_new:N { c }
%    \end{macrocode}
%
% \begin{macro}{\box_clear:N, \box_clear:c}
% \begin{macro}{\box_gclear:N, \box_gclear:c}
% \testfile*
%   Clear a \meta{box} register.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \box_clear:N #1
  { \box_set_eq:NN  #1 \c_empty_box }
\cs_new_protected_nopar:Npn \box_gclear:N #1
  { \box_gset_eq:NN #1 \c_empty_box }
\cs_generate_variant:Nn \box_clear:N  { c }
\cs_generate_variant:Nn \box_gclear:N { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\box_clear_new:N, \box_clear_new:c}
% \begin{macro}{\box_gclear_new:N, \box_gclear_new:c}
% \testfile*
%   Clear or new.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \box_clear_new:N #1
  {
    \cs_if_exist:NTF #1
      { \box_set_eq:NN #1 \c_empty_box }
      { \box_new:N #1 }
  }
\cs_new_protected_nopar:Npn \box_gclear_new:N #1
  {
    \cs_if_exist:NTF #1
      { \box_gset_eq:NN #1 \c_empty_box }
      { \box_new:N #1 }
  }
\cs_generate_variant:Nn \box_clear_new:N  { c }
\cs_generate_variant:Nn \box_gclear_new:N { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
%  \begin{macro}
%    {\box_set_eq:NN, \box_set_eq:cN, \box_set_eq:Nc, \box_set_eq:cc}
% \testfile*
%  \begin{macro}
%    {\box_gset_eq:NN, \box_gset_eq:cN, \box_gset_eq:Nc, \box_gset_eq:cc}
% \testfile*
%   Assigning the contents of a box to be another box.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \box_set_eq:NN #1#2
  { \tex_setbox:D #1 \tex_copy:D #2 }
\cs_new_protected_nopar:Npn \box_gset_eq:NN
  { \pref_global:D \box_set_eq:NN }
\cs_generate_variant:Nn \box_set_eq:NN  { cN , Nc , cc }
\cs_generate_variant:Nn \box_gset_eq:NN { cN , Nc , cc }
%    \end{macrocode}
%  \end{macro}
%  \end{macro}
%
% \begin{macro}
%   {
%     \box_set_eq_clear:NN, \box_set_eq_clear:cN,
%     \box_set_eq_clear:Nc, \box_set_eq_clear:cc
%   }
% \testfile*
% \begin{macro}
%   {
%     \box_gset_eq_clear:NN, \box_gset_eq_clear:cN,
%     \box_gset_eq_clear:Nc, \box_gset_eq_clear:cc
%   }
% \testfile*
%    Assigning the contents of a box to be another box.
%    This clears the second box globally (that's how \TeX{} does it).
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \box_set_eq_clear:NN #1#2
  { \tex_setbox:D #1 \tex_box:D #2 }
\cs_new_protected_nopar:Npn \box_gset_eq_clear:NN
  { \pref_global:D  \box_set_eq_clear:NN }
\cs_generate_variant:Nn \box_set_eq_clear:NN  { cN , Nc , cc }
\cs_generate_variant:Nn \box_gset_eq_clear:NN { cN , Nc , cc }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \subsection{Measuring and setting box dimensions}
%
% \begin{macro}{\box_ht:N,\box_ht:c}
% \begin{macro}{\box_dp:N,\box_dp:c}
% \begin{macro}{\box_wd:N,\box_wd:c}
% \testfile*
%    Accessing the height, depth, and width of a \meta{box} register.
%    \begin{macrocode}
\cs_new_eq:NN \box_ht:N \tex_ht:D
\cs_new_eq:NN \box_dp:N \tex_dp:D
\cs_new_eq:NN \box_wd:N \tex_wd:D
\cs_generate_variant:Nn \box_ht:N { c }
\cs_generate_variant:Nn \box_dp:N { c }
\cs_generate_variant:Nn \box_wd:N { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\box_set_ht:Nn, \box_set_ht:cn}
% \begin{macro}{\box_set_dp:Nn, \box_set_dp:cn}
% \begin{macro}{\box_set_wd:Nn, \box_set_wd:cn}
%   Measuring is easy: all primitive work. These primitives are not
%   expandable, so the derived functions are not either.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \box_set_dp:Nn #1#2
  { \box_dp:N #1 \dim_eval:w #2 \dim_eval_end: }
\cs_new_protected_nopar:Npn \box_set_ht:Nn #1#2
  { \box_ht:N #1 \dim_eval:w #2 \dim_eval_end: }
\cs_new_protected_nopar:Npn \box_set_wd:Nn #1#2
  { \box_wd:N #1 \dim_eval:w #2 \dim_eval_end: }
\cs_generate_variant:Nn \box_set_ht:Nn { c }
\cs_generate_variant:Nn \box_set_dp:Nn { c }
\cs_generate_variant:Nn \box_set_wd:Nn { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \subsection{Using boxes}
%
% \begin{macro}{\box_use_clear:N, \box_use_clear:c}
% \begin{macro}{\box_use:N, \box_use:c}
%   Using a \meta{box}. These are just \TeX{} primitives with meaningful
%   names.
%    \begin{macrocode}
\cs_new_eq:NN \box_use_clear:N \tex_box:D
\cs_new_eq:NN \box_use:N \tex_copy:D
\cs_generate_variant:Nn \box_use_clear:N { c }
\cs_generate_variant:Nn \box_use:N { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\box_move_left:nn,\box_move_right:nn}
% \begin{macro}{\box_move_up:nn,\box_move_down:nn}
% \testfile*
%   Move box material in different directions.
%    \begin{macrocode}
\cs_new_protected:Npn \box_move_left:nn #1#2
  { \tex_moveleft:D \dim_eval:w #1 \dim_eval_end: #2 }
\cs_new_protected:Npn \box_move_right:nn #1#2
  { \tex_moveright:D \dim_eval:w #1 \dim_eval_end: #2 }
\cs_new_protected:Npn \box_move_up:nn #1#2
  { \tex_raise:D \dim_eval:w #1 \dim_eval_end: #2 }
\cs_new_protected:Npn \box_move_down:nn #1#2
  { \tex_lower:D \dim_eval:w #1 \dim_eval_end: #2 }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \subsection{Box conditionals}
%
% \begin{macro}{\if_hbox:N}
% \begin{macro}{\if_vbox:N}
% \begin{macro}{\if_box_empty:N}
%  \testfile*
%    The primitives for testing if a \meta{box} is empty/void or which
%    type of box it is.
%    \begin{macrocode}
\cs_new_eq:NN \if_hbox:N      \tex_ifhbox:D
\cs_new_eq:NN \if_vbox:N      \tex_ifvbox:D
\cs_new_eq:NN \if_box_empty:N \tex_ifvoid:D
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}[pTF]{\box_if_horizontal:N,\box_if_horizontal:c}
% \testfile*
% \begin{macro}[pTF]{\box_if_vertical:N,\box_if_vertical:c}
% \testfile*
%    \begin{macrocode}
\prg_new_conditional:Npnn \box_if_horizontal:N #1 { p , T , F , TF }
  { \if_hbox:N #1 \prg_return_true: \else: \prg_return_false: \fi: }
\prg_new_conditional:Npnn \box_if_vertical:N #1 { p , T , F , TF }
  { \if_vbox:N #1 \prg_return_true: \else: \prg_return_false: \fi: }
\cs_generate_variant:Nn \box_if_horizontal_p:N { c }
\cs_generate_variant:Nn \box_if_horizontal:NT  { c }
\cs_generate_variant:Nn \box_if_horizontal:NF  { c }
\cs_generate_variant:Nn \box_if_horizontal:NTF { c }
\cs_generate_variant:Nn \box_if_vertical_p:N { c }
\cs_generate_variant:Nn \box_if_vertical:NT  { c }
\cs_generate_variant:Nn \box_if_vertical:NF  { c }
\cs_generate_variant:Nn \box_if_vertical:NTF { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}[pTF]{\box_if_empty:N, \box_if_empty:c}
% \testfile*
%   Testing if a \meta{box} is empty/void.
%    \begin{macrocode}
\prg_new_conditional:Npnn \box_if_empty:N #1 { p , T , F , TF }
  { \if_box_empty:N #1 \prg_return_true: \else: \prg_return_false: \fi: }
\cs_generate_variant:Nn \box_if_empty_p:N { c }
\cs_generate_variant:Nn \box_if_empty:NT  { c }
\cs_generate_variant:Nn \box_if_empty:NF  { c }
\cs_generate_variant:Nn \box_if_empty:NTF { c }
%    \end{macrocode}
%  \end{macro}
%  \end{macro}
%
% \subsection{The last box inserted}
%
% \begin{variable}{\l_last_box}
%    A different name for this read-only primitive.
%    \begin{macrocode}
\cs_new_eq:NN \l_last_box \tex_lastbox:D
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}{\box_set_to_last:N, \box_set_to_last:c}
% \begin{macro}{\box_gset_to_last:N, \box_gset_to_last:c}
% \testfile*
%    Set a box to the previous box.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \box_set_to_last:N #1
  { \tex_setbox:D #1 \l_last_box }
\cs_new_protected_nopar:Npn \box_gset_to_last:N
  { \pref_global:D \box_set_to_last:N }
\cs_generate_variant:Nn \box_set_to_last:N  { c }
\cs_generate_variant:Nn \box_gset_to_last:N { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \subsection{Constant boxes}
%
%  \begin{variable}{\c_empty_box}
%    \begin{macrocode}
%<*package>
\cs_new_eq:NN \c_empty_box \voidb@x
%</package>
%<*initex>
\box_new:N \c_empty_box
%</initex>
%    \end{macrocode}
% \end{variable}
%
% \subsection{Scratch boxes}
%
%  \begin{variable}{\l_tmpa_box, \l_tmpb_box}
%    \begin{macrocode}
%<*package>
\cs_new_eq:NN \l_tmpa_box \@tempboxa
%</package>
%<*initex>
\box_new:N \l_tmpa_box
%</initex>
\box_new:N \l_tmpb_box
%    \end{macrocode}
% \end{variable}
%
% \subsection{Viewing box contents}
%
% \begin{macro}{\box_show:N,\box_show:c}
% \testfile*
%   Show the contents of a box and write it into the log file.
%    \begin{macrocode}
\cs_new_eq:NN \box_show:N \tex_showbox:D
\cs_generate_variant:Nn \box_show:N { c }
%    \end{macrocode}
%  \end{macro}
%
%  \subsection{Horizontal mode boxes}
%
% \begin{macro}{\hbox:n}
% \testfile{m3box002.lvt}
%   Put a horizontal box directly into the input stream.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \hbox:n { \tex_hbox:D \scan_stop: }
%    \end{macrocode}
%  \end{macro}
%
% \begin{macro}{\hbox_set:Nn,\hbox_set:cn}
% \begin{macro}{\hbox_gset:Nn,\hbox_gset:cn}
% \testfile*
%    \begin{macrocode}
\cs_new_protected:Npn \hbox_set:Nn #1#2 { \tex_setbox:D #1 \tex_hbox:D {#2} }
\cs_new_protected_nopar:Npn \hbox_gset:Nn { \pref_global:D \hbox_set:Nn }
\cs_generate_variant:Nn \hbox_set:Nn { c }
\cs_generate_variant:Nn \hbox_gset:Nn { c }
%    \end{macrocode}
%  \end{macro}
%  \end{macro}
%
% \begin{macro}{\hbox_set_to_wd:Nnn,\hbox_set_to_wd:cnn}
% \begin{macro}{\hbox_gset_to_wd:Nnn,\hbox_gset_to_wd:cnn}
% \testfile*
%   Storing material in a horizontal box with a specified width.
%    \begin{macrocode}
\cs_new_protected:Npn \hbox_set_to_wd:Nnn #1#2#3
  { \tex_setbox:D #1 \tex_hbox:D to \dim_eval:w #2 \dim_eval_end: {#3} }
\cs_new_protected_nopar:Npn \hbox_gset_to_wd:Nnn
  { \pref_global:D \hbox_set_to_wd:Nnn }
\cs_generate_variant:Nn \hbox_set_to_wd:Nnn { c }
\cs_generate_variant:Nn \hbox_gset_to_wd:Nnn {cnn}
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\hbox_set_inline_begin:N, \hbox_set_inline_begin:c}
% \begin{macro}{\hbox_gset_inline_begin:N, \hbox_gset_inline_begin:c}
% \begin{macro}{\hbox_set_inline_end:}
% \begin{macro}{\hbox_gset_inline_end:}
% \testfile*
%    Storing material in a horizontal box. This type is useful in
%    environment definitions.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \hbox_set_inline_begin:N  #1
  { \tex_setbox:D #1 \tex_hbox:D \c_group_begin_token }
\cs_new_protected_nopar:Npn \hbox_gset_inline_begin:N
  { \pref_global:D \hbox_set_inline_begin:N }
\cs_generate_variant:Nn \hbox_set_inline_begin:N  { c }
\cs_generate_variant:Nn \hbox_gset_inline_begin:N { c }
\cs_new_eq:NN \hbox_set_inline_end:  \c_group_end_token
\cs_new_eq:NN \hbox_gset_inline_end: \c_group_end_token
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
%  \begin{macro}{\hbox_to_wd:nn}
%  \begin{macro}{\hbox_to_zero:n}
%  \testfile*
%   Put a horizontal box directly into the input stream.
%    \begin{macrocode}
\cs_new_protected:Npn \hbox_to_wd:nn #1#2
   { \tex_hbox:D to \dim_eval:w #1 \dim_eval_end: {#2} }
\cs_new_protected:Npn \hbox_to_zero:n #1 { \tex_hbox:D to \c_zero_skip {#1} }
%    \end{macrocode}
%  \end{macro}
%  \end{macro}
%
% \begin{macro}{\hbox_overlap_left:n,}
% \begin{macro}{\hbox_overlap_right:n}
%   Put a zero-sized box with the contents pushed against one side (which
%   makes it stick out on the other) directly into the input stream.
%    \begin{macrocode}
\cs_new_protected:Npn \hbox_overlap_left:n  #1
  { \hbox_to_zero:n { \tex_hss:D #1 } }
\cs_new_protected:Npn \hbox_overlap_right:n #1
  { \hbox_to_zero:n { #1 \tex_hss:D } }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\hbox_unpack:N, \hbox_unpack:c}
% \begin{macro}{\hbox_unpack_clear:N, \hbox_unpack_clear:c}
% \testfile*
%   Unpacking a box and if requested also clear it.
%    \begin{macrocode}
\cs_new_eq:NN \hbox_unpack:N \tex_unhcopy:D
\cs_new_eq:NN \hbox_unpack_clear:N \tex_unhbox:D
\cs_generate_variant:Nn \hbox_unpack:N { c }
\cs_generate_variant:Nn \hbox_unpack_clear:N { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
%  \subsection{Vertical mode boxes}
%
% \begin{macro}{\vbox:n}
% \TestFiles{m3box003.lvt}
% \begin{macro}{\vbox:n\vbox_top:n}
% \TestFiles{m3box003.lvt}
%   Put a vertical box directly into the input stream.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \vbox:n { \tex_vbox:D \scan_stop: }
\cs_new_protected_nopar:Npn \vbox_top:n { \tex_vtop:D \scan_stop: }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\vbox_to_ht:nn,\vbox_to_zero:n}
% \begin{macro}{\vbox_to_ht:nn,\vbox_to_zero:n}
% \testfile*
%   Put a vertical box directly into the input stream.
%    \begin{macrocode}
\cs_new_protected:Npn \vbox_to_ht:nn #1#2
  { \tex_vbox:D to \dim_eval:w #1 \dim_eval_end: {#2} }
\cs_new_protected:Npn \vbox_to_zero:n #1 { \tex_vbox:D to \c_zero_dim {#1} }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\vbox_set:Nn, \vbox_set:cn}
% \begin{macro}{\vbox_gset:Nn, \vbox_gset:cn}
% \testfile*
%   Storing material in a vertical box with a natural height.
%    \begin{macrocode}
\cs_new_protected:Npn \vbox_set:Nn #1#2 { \tex_setbox:D #1 \tex_vbox:D {#2} }
\cs_new_protected_nopar:Npn \vbox_gset:Nn  { \pref_global:D \vbox_set:Nn }
\cs_generate_variant:Nn \vbox_set:Nn  { c }
\cs_generate_variant:Nn \vbox_gset:Nn { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\vbox_set_top:Nn, \vbox_set_top:cn}
% \begin{macro}{\vbox_gset_top:Nn, \vbox_gset_top:cn}
% \testfile*
%   Storing material in a vertical box with a natural height and reference
%   point at the baseline of the first object in the box.
%    \begin{macrocode}
\cs_new_protected:Npn \vbox_set_top:Nn #1#2
  { \tex_setbox:D #1 \tex_vtop:D {#2} }
\cs_new_protected_nopar:Npn \vbox_gset_top:Nn
  { \pref_global:D \vbox_set_top:Nn }
\cs_generate_variant:Nn \vbox_set_top:Nn { c }
\cs_generate_variant:Nn \vbox_gset_top:Nn { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\vbox_set_to_ht:Nnn,\vbox_set_to_ht:cnn}
% \begin{macro}{\vbox_gset_to_ht:Nnn,\vbox_gset_to_ht:cnn}
%  \testfile*
%  Storing material in a vertical box with a specified height.
%    \begin{macrocode}
\cs_new_protected:Npn \vbox_set_to_ht:Nnn #1#2#3
  { \tex_setbox:D #1 \tex_vbox:D to \dim_eval:w #2 \dim_eval_end: {#3} }
\cs_new_protected_nopar:Npn \vbox_gset_to_ht:Nnn
  { \pref_global:D \vbox_set_to_ht:Nnn }
\cs_generate_variant:Nn \vbox_set_to_ht:Nnn  { c }
\cs_generate_variant:Nn \vbox_gset_to_ht:Nnn { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\vbox_set_inline_begin:N, \vbox_set_inline_begin:c}
% \begin{macro}{\vbox_gset_inline_begin:N, \vbox_gset_inline_begin:c}
% \begin{macro}{\vbox_set_inline_end:}
% \begin{macro}{\vbox_gset_inline_end:}
% \testfile*
%   Storing material in a vertical box. This type is useful in
%   environment definitions.
%    \begin{macrocode}
\cs_new_nopar:Npn \vbox_set_inline_begin:N #1
  { \tex_setbox:D #1 \tex_vbox:D \c_group_begin_token }
\cs_new_protected_nopar:Npn \vbox_gset_inline_begin:N
  { \pref_global:D \vbox_set_inline_begin:N }
\cs_generate_variant:Nn \vbox_set_inline_begin:N  { c }
\cs_generate_variant:Nn \vbox_gset_inline_begin:N { c }
\cs_new_eq:NN \vbox_set_inline_end:  \c_group_end_token
\cs_new_eq:NN \vbox_gset_inline_end: \c_group_end_token
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\vbox_unpack:N, \vbox_unpack:c}
% \begin{macro}{\vbox_unpack_clear:N, \vbox_unpack_clear:c}
% \testfile*
%   Unpacking a box and if requested also clear it.
%    \begin{macrocode}
\cs_new_eq:NN \vbox_unpack:N \tex_unvcopy:D
\cs_new_eq:NN \vbox_unpack_clear:N \tex_unvbox:D
\cs_generate_variant:Nn \vbox_unpack:N { c }
\cs_generate_variant:Nn \vbox_unpack_clear:N { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\vbox_set_split_to_ht:NNn}
% \testfile*
%   Splitting a vertical box in two.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \vbox_set_split_to_ht:NNn #1#2#3
  { \tex_setbox:D #1 \tex_vsplit:D #2 to \dim_eval:w #3 \dim_eval_end: }
%    \end{macrocode}
% \end{macro}
%
%    \begin{macrocode}
%</initex|package>
%    \end{macrocode}
%
% \end{implementation}
%
% \PrintIndex
