path: root/Master/texmf-dist/source/latex/l3kernel/l3box.dtx

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%% File: l3box.dtx Copyright (C) 2005-2011 The LaTeX3 Project
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%
%<*driver|package>
\RequirePackage{l3names}
\GetIdInfo$Id: l3box.dtx 2896 2011-10-09 20:36:50Z 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 \tn{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 \tn{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 operation 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 operation 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}[EXP]{\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 \tn{dp}.
%   \end{texnote}
% \end{function}
%
% \begin{function}[EXP]{\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 \tn{ht}.
%   \end{texnote}
% \end{function}
%
% \begin{function}[EXP]{\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 \tn{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{Affine transformations}
%
% Affine transformations are changes which (informally) preserve straight
% lines. Simple translations are affine transformations, but are better handled
% in \TeX{} by doing the translation first, then inserting an unmodified box.
% On the other hand, rotation and resizing of boxed material can best be
% handled by modifying boxes. These transformations are described here.
%
% \begin{function}[added = 2011-09-02]{\box_resize:Nnn, \box_resize:cnn}
%   \begin{syntax}
%     \cs{box_resize:Nnn} \meta{box} \Arg{x-size} \Arg{y-size}
%   \end{syntax}
%   Resize the \meta{box} to \meta{x-size} horizontally and \meta{y-size}
%   vertically (both of the sizes are dimension expressions).
%   The \meta{y-size} is the vertical size (height plus depth) of
%   the box. The updated \meta{box} will be an hbox, irrespective of the nature
%   of the \meta{box}  before the resizing is applied. Negative sizes will
%   cause the material in the \meta{box} to be reversed in direction, but the
%   reference point of the \meta{box} will be unchanged. The resizing applies
%   within the current \TeX{} group level.
%
%   \textbf{This function is experimental}
% \end{function}
%
% \begin{function}[added = 2011-09-02]
%   {\box_resize_to_ht_plus_dp:Nn, \box_resize_to_ht_plus_dp:cn}
%   \begin{syntax}
%     \cs{box_resize_to_ht_plus_dp:Nnn} \meta{box} \Arg{y-size}
%   \end{syntax}
%   Resize the \meta{box} to \meta{y-size} vertically, scaling the horizontal
%   size by the same amount (\meta{y-size} is a dimension expression).
%   The \meta{y-size} is the vertical size (height plus depth) of
%   the box.
%   The updated \meta{box} will be an hbox, irrespective of the nature
%   of the \meta{box}  before the resizing is applied. A negative size will
%   cause the material in the \meta{box} to be reversed in direction, but the
%   reference point of the \meta{box} will be unchanged. The resizing applies
%   within the current \TeX{} group level.
%
%  \textbf{This function is experimental}
% \end{function}
%
% \begin{function}[added = 2011-09-02]
%   {\box_resize_to_wd:Nn, \box_resize_to_wd:cn}
%   \begin{syntax}
%     \cs{box_resize_to_wd:Nnn} \meta{box} \Arg{x-size}
%   \end{syntax}
%   Resize the \meta{box} to \meta{x-size} horizontally, scaling the vertical
%   size by the same amount (\meta{x-size} is a dimension expression).
%   The updated \meta{box} will be an hbox, irrespective of the nature
%   of the \meta{box}  before the resizing is applied. A negative size will
%   cause the material in the \meta{box} to be reversed in direction, but the
%   reference point of the \meta{box} will be unchanged. The resizing applies
%   within the current \TeX{} group level.
%
%  \textbf{This function is experimental}
% \end{function}
%
% \begin{function}[added = 2011-09-02]{\box_rotate:Nn, \box_rotate:cn}
%   \begin{syntax}
%     \cs{box_rotate:Nn} \meta{box} \Arg{angle}
%   \end{syntax}
%   Rotates the \meta{box} by \meta{angle} (in degrees) anti-clockwise about
%   its reference point. The reference point of the updated box will be moved
%   horizontally such that it is at the left side of the smallest rectangle
%   enclosing the rotated material.
%   The updated \meta{box} will be an hbox, irrespective of the nature
%   of the \meta{box} before the rotation is applied. The rotation applies
%   within the current \TeX{} group level.
%
%  \textbf{This function is experimental}
% \end{function}
%
% \begin{function}[added = 2011-09-02]{\box_scale:Nnn, \box_scale:cnn}
%   \begin{syntax}
%     \cs{box_scale:Nnn} \meta{box} \Arg{x-scale} \Arg{y-scale}
%   \end{syntax}
%   Scales the \meta{box} by factors \meta{x-scale} and \meta{y-scale} in
%   the horizontal and vertical directions, respectively (both scales are
%   integer expressions). The updated \meta{box} will be an hbox, irrespective
%   of the nature of the \meta{box} before the scaling is applied. Negative
%   scalings will cause the material in the \meta{box} to be reversed in
%   direction, but the reference point of the \meta{box} will be unchanged.
%   The scaling applies within the current \TeX{} group level.
%
%  \textbf{This function is experimental}
% \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}).
% \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.
% \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.
% \end{function}
%
% \section{The last box inserted}
% 
% \begin{function}
%   {
%     \box_set_to_last:N,  \box_set_to_last:c,
%     \box_gset_to_last:N, \box_gset_to_last:c
%   }
%   \begin{syntax}
%     \cs{box_set_to_last:N} \meta{box}
%   \end{syntax}
%   Sets the \meta{box} equal to the last item (box) added to the current
%   partial list, removing the item from the list at the same time. When
%   applied to the main vertical list, the \meta{box} will always be void as
%   it is not possible to recover the last added item.
% \end{function}
%
% \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 \tn{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_box, \l_tmpb_box}
%   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 \tn{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 \tn{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:Nw, \hbox_set:cw,
%     \hbox_set_end:
%   }
%   \begin{syntax}
%     \cs{hbox_set:Nw} \meta{box} \meta{contents} \cs{hbox_set_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:Nw, \hbox_gset:cw,
%     \hbox_gset_end:
%   }
%   \begin{syntax}
%     \cs{hbox_gset:Nw} \meta{box} \meta{contents} \cs{hbox_gset_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 \tn{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 \tn{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 latter 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 \tn{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 \tn{vtop}.
%   \end{texnote}
% \end{function}
%
% \begin{function}{\vbox_to_ht:nn}
%   \begin{syntax}
%     \cs{vbox_to_ht:nn} \Arg{dimexpr} \Arg{contents}
%   \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:Nw, \vbox_set:cw,
%    \vbox_set_end:
%  }
%    \begin{syntax}
%      \cs{vbox_begin:Nw} \meta{box} \meta{contents} \cs{vbox_set_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:Nw, \vbox_gset:cw,
%    \vbox_gset_end:
%  }
%    \begin{syntax}
%      \cs{vbox_gset:Nw} \meta{box} \meta{contents} \cs{vbox_gset_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 \tn{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 \tn{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 \tn{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 \tn{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 \tn{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 \tn{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:
%</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}
%<*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
  { \tex_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
  { \tex_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
  { \tex_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 { \tex_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
  { \tex_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 { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\hbox_set:Nw, \hbox_set:cw}
% \begin{macro}{\hbox_gset:Nw, \hbox_gset:cw}
% \begin{macro}{\hbox_set_end:, \hbox_gset_end:}
% \testfile*
%    Storing material in a horizontal box. This type is useful in
%    environment definitions.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \hbox_set:Nw  #1
  { \tex_setbox:D #1 \tex_hbox:D \c_group_begin_token }
\cs_new_protected_nopar:Npn \hbox_gset:Nw
  { \tex_global:D \hbox_set:Nw }
\cs_generate_variant:Nn \hbox_set:Nw  { c }
\cs_generate_variant:Nn \hbox_gset:Nw { c }
\cs_new_eq:NN \hbox_set_end:  \c_group_end_token
\cs_new_eq:NN \hbox_gset_end: \c_group_end_token
%    \end{macrocode}
% \end{macro}
% \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:,\hbox_gset_inline_end:}
% \testfile*
% Renamed September 2011.
%    \begin{macrocode}
\cs_new_eq:NN \hbox_set_inline_begin:N  \hbox_set:Nw
\cs_new_eq:NN \hbox_set_inline_begin:c  \hbox_set:cw
\cs_new_eq:NN \hbox_set_inline_end:     \hbox_set_end:
\cs_new_eq:NN \hbox_gset_inline_begin:N \hbox_gset:Nw
\cs_new_eq:NN \hbox_gset_inline_begin:c \hbox_gset:cw
\cs_new_eq:NN \hbox_gset_inline_end:    \hbox_gset_end:
%    \end{macrocode}
% \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_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  { \tex_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
  { \tex_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
  { \tex_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:Nw, \vbox_set:cw}
% \begin{macro}{\vbox_gset:Nw, \vbox_gset:cw}
% \begin{macro}{\vbox_set_end:, \vbox_gset_end:}
% \testfile*
%   Storing material in a vertical box. This type is useful in
%   environment definitions.
%    \begin{macrocode}
\cs_new_nopar:Npn \vbox_set:Nw #1
  { \tex_setbox:D #1 \tex_vbox:D \c_group_begin_token }
\cs_new_protected_nopar:Npn \vbox_gset:Nw
  { \tex_global:D \vbox_set:Nw }
\cs_generate_variant:Nn \vbox_set:Nw  { c }
\cs_generate_variant:Nn \vbox_gset:Nw { c }
\cs_new_eq:NN \vbox_set_end:  \c_group_end_token
\cs_new_eq:NN \vbox_gset_end: \c_group_end_token
%    \end{macrocode}
% \end{macro}
% \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*
% Renamed September 2011.
%    \begin{macrocode}
\cs_new_eq:NN \vbox_set_inline_begin:N  \vbox_set:Nw
\cs_new_eq:NN \vbox_set_inline_begin:c  \vbox_set:cw
\cs_new_eq:NN \vbox_set_inline_end:  \vbox_set_end:
\cs_new_eq:NN \vbox_gset_inline_begin:N \vbox_gset:Nw
\cs_new_eq:NN \vbox_gset_inline_begin:c \vbox_gset:cw
\cs_new_eq:NN \vbox_gset_inline_end: \vbox_gset_end:
%    \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}
%
% \subsection{Affine transformations}
%
% \begin{variable}{\l_box_angle_fp}
%   When rotating boxes, the angle itself may be needed by the
%   engine-dependent code. This is done using the \pkg{fp} module so
%   that the value is tidied up properly.
%    \begin{macrocode}
\fp_new:N \l_box_angle_fp
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_box_cos_fp, \l_box_sin_fp}
%   These are used to hold the calculated sine and cosine values while
%   carrying out a rotation.
%    \begin{macrocode}
\fp_new:N \l_box_cos_fp
\fp_new:N \l_box_sin_fp
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}
%   {\l_box_top_dim, \l_box_bottom_dim, \l_box_left_dim, \l_box_right_dim}
%   These are the positions of the four edges of a box before
%   manipulation.
%    \begin{macrocode}
\dim_new:N \l_box_top_dim
\dim_new:N \l_box_bottom_dim
\dim_new:N \l_box_left_dim
\dim_new:N \l_box_right_dim
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}
%  {
%    \l_box_top_new_dim,  \l_box_bottom_new_dim ,
%    \l_box_left_new_dim, \l_box_right_new_dim
%  }
%   These are the positions of the four edges of a box after
%   manipulation.
%    \begin{macrocode}
\dim_new:N \l_box_top_new_dim
\dim_new:N \l_box_bottom_new_dim
\dim_new:N \l_box_left_new_dim
\dim_new:N \l_box_right_new_dim
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_box_tmp_box, \l_box_tmp_fp}
%   Scratch space.
%    \begin{macrocode}
\box_new:N \l_box_tmp_box
\fp_new:N \l_box_tmp_fp
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_box_x_fp, \l_box_y_fp, \l_box_x_new_fp, \l_box_y_new_fp}
%   Used as the input and output values for a point when manipulation the
%   location.
%    \begin{macrocode}
\fp_new:N \l_box_x_fp
\fp_new:N \l_box_y_fp
\fp_new:N \l_box_x_new_fp
\fp_new:N \l_box_y_new_fp
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}{\box_rotate:Nn}
% \begin{macro}[aux]{\box_rotate_aux:N}
% \begin{macro}[aux]{\box_rotate_set_sin_cos:}
% \begin{macro}[aux]{\box_rotate_x:nnN, \box_rotate_y:nnN}
% \begin{macro}[aux]
%   {
%     \box_rotate_quadrant_one:,   \box_rotate_quadrant_two:,
%     \box_rotate_quadrant_three:, \box_rotate_quadrant_four:
%   }
%   Rotation of a box starts with working out the relevant sine and
%   cosine. There is then a check to avoid doing any real work for the
%   trivial rotation.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \box_rotate:Nn #1#2
  {
    \hbox_set:Nn #1
      {
        \group_begin:
          \fp_set:Nn \l_box_angle_fp {#2}
          \box_rotate_set_sin_cos:
          \fp_compare:NNNTF \l_box_sin_fp = \c_zero_fp
            {
               \fp_compare:NNNTF \l_box_cos_fp = \c_one_fp
                 { \box_use:N #1 }
                 { \box_rotate_aux:N #1 }
            }
            { \box_rotate_aux:N #1 }
        \group_end:
    }
  }
%    \end{macrocode}
%   The edges of the box are then recorded: the left edge will
%   always be at zero. Rotation of the four edges then takes place: this is
%   most efficiently done on a quadrant by quadrant basis.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \box_rotate_aux:N #1
  {
    \dim_set:Nn \l_box_top_dim    {  \box_ht:N #1 }
    \dim_set:Nn \l_box_bottom_dim { -\box_dp:N #1 }
    \dim_set:Nn \l_box_right_dim  {  \box_wd:N #1 }
    \dim_zero:N \l_box_left_dim
%    \end{macrocode}
%   The next step is to work out the $x$ and $y$ coordinates of vertices of
%   the rotated box in relation to its original coordinates. The box can be
%   visualized with vertices $B$, $C$, $D$ and $E$ is illustrated
%   (Figure~\ref{fig:rotation}). The vertex $O$ is the reference point on the
%   baseline, and in this implementation is also the centre of rotation.
%   \begin{figure}
%     \centering
%     \setlength{\unitlength}{3pt}^^A
%     \begin{picture}(34,36)(12,44)
%       \thicklines
%       \put(20,52){\dashbox{1}(20,21){}}
%       \put(20,80){\line(0,-1){36}}
%       \put(12,58){\line(1, 0){34}}
%       \put(41,59){A}
%       \put(40,74){B}
%       \put(21,74){C}
%       \put(21,49){D}
%       \put(40,49){E}
%       \put(21,59){O}
%     \end{picture}
%     \caption{Co-ordinates of a box prior to rotation.}
%     \label{fig:rotation}
%   \end{figure}
%   The formulae are, for a point $P$ and angle $\alpha$:
%   \[
%     \begin{array}{l}
%       P'_x = P_x - O_x \\
%       P'_y = P_y - O_y \\
%       P''_x =  ( P'_x \cos(\alpha)) - ( P'_y \sin(\alpha) ) \\
%       P''_y =  ( P'_x \sin(\alpha)) + ( P'_y \cos(\alpha) ) \\
%       P'''_x = P''_x + O_x + L_x \\
%       P'''_y = P''_y + O_y
%    \end{array}
%   \]
%   The \enquote{extra} horizontal translation $L_x$ at the end is calculated
%   so that the leftmost point of the resulting box has $x$-coordinate $0$.
%   This is desirable as \TeX{} boxes must have the reference point at
%   the left edge of the box. (As $O$ is always $(0,0)$, this part of the
%   calculation is omitted here.)
%    \begin{macrocode}
    \fp_compare:NNNTF \l_box_sin_fp > \c_zero_fp
      {
        \fp_compare:NNNTF \l_box_cos_fp > \c_zero_fp
          { \box_rotate_quadrant_one: }
          { \box_rotate_quadrant_two: }
      }
      {
        \fp_compare:NNNTF \l_box_cos_fp < \c_zero_fp
          { \box_rotate_quadrant_three: }
          { \box_rotate_quadrant_four: }
      }
%    \end{macrocode}
%   The position of the box edges are now known, but the box at this
%   stage be misplaced relative to the current \TeX{} reference point. So the
%   content of the box is moved such that the reference point of the
%   rotated box will be in the same place as the original.
%    \begin{macrocode}
    \hbox_set:Nn \l_box_tmp_box { \box_use:N #1 }
    \hbox_set:Nn \l_box_tmp_box
      {
        \tex_kern:D -\l_box_left_new_dim
        \hbox:n
          {
            \driver_box_rotate_begin:
            \box_use:N \l_box_tmp_box
            \driver_box_rotate_end:
          }
      }
%    \end{macrocode}
%   Tidy up the size of the box so that the material is actually inside
%   the bounding box. The result can then be used to reset the original
%   box.
%    \begin{macrocode}
    \box_set_ht:Nn \l_box_tmp_box {  \l_box_top_new_dim }
    \box_set_dp:Nn \l_box_tmp_box { -\l_box_bottom_new_dim }
    \box_set_wd:Nn \l_box_tmp_box
      { \l_box_right_new_dim - \l_box_left_new_dim }
    \box_use:N \l_box_tmp_box
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%   A simple conversion from degrees to radians followed by calculation
%   of the sine and cosine.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \box_rotate_set_sin_cos:
  {
    \fp_set_eq:NN \l_box_tmp_fp \l_box_angle_fp
    \fp_div:Nn \l_box_tmp_fp { 180 }
    \fp_mul:Nn \l_box_tmp_fp { \c_pi_fp }
    \fp_sin:Nn \l_box_sin_fp { \l_box_tmp_fp }
    \fp_cos:Nn \l_box_cos_fp { \l_box_tmp_fp }
  }
%    \end{macrocode}
%   These functions take a general point $(|#1|, |#2|)$ and rotate its
%   location about the origin, using the previously-set sine and cosine
%   values. Each function gives only one component of the location of the
%   updated point. This is because for rotation of a box each step needs
%   only one value, and so performance is gained by avoiding working
%   out both $x'$ and $y'$ at the same time. Contrast this with
%   the equivalent function in the \pkg{l3coffins} module, where both parts
%   are needed.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \box_rotate_x:nnN #1#2#3
  {
    \fp_set_from_dim:Nn \l_box_x_fp {#1}
    \fp_set_from_dim:Nn \l_box_y_fp {#2}
    \fp_set_eq:NN \l_box_x_new_fp \l_box_x_fp
    \fp_set_eq:NN \l_box_tmp_fp   \l_box_y_fp
    \fp_mul:Nn \l_box_x_new_fp { \l_box_cos_fp }
    \fp_mul:Nn \l_box_tmp_fp   { \l_box_sin_fp }
    \fp_sub:Nn \l_box_x_new_fp { \l_box_tmp_fp }
    \dim_set:Nn #3 { \fp_to_dim:N \l_box_x_new_fp }
  }
\cs_new_protected_nopar:Npn \box_rotate_y:nnN #1#2#3
  {
    \fp_set_from_dim:Nn \l_box_x_fp {#1}
    \fp_set_from_dim:Nn \l_box_y_fp {#2}
    \fp_set_eq:NN \l_box_y_new_fp \l_box_y_fp
    \fp_set_eq:NN \l_box_tmp_fp   \l_box_x_fp
    \fp_mul:Nn \l_box_y_new_fp { \l_box_cos_fp }
    \fp_mul:Nn \l_box_tmp_fp   { \l_box_sin_fp }
    \fp_add:Nn \l_box_y_new_fp { \l_box_tmp_fp }
    \dim_set:Nn #3 { \fp_to_dim:N \l_box_y_new_fp }
}
%    \end{macrocode}
%   Rotation of the edges is done using a different formula for each
%   quadrant. In every case, the top and bottom edges only need the
%   resulting $y$-values, whereas the left and right edges need the
%   $x$-values. Each case is a question of picking out which corner
%   ends up at with the maximum top, bottom, left and right value. Doing
%   this by hand means a lot less calculating and avoids lots of
%   comparisons.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \box_rotate_quadrant_one:
  {
    \box_rotate_y:nnN \l_box_right_dim \l_box_top_dim
      \l_box_top_new_dim
    \box_rotate_y:nnN \l_box_left_dim  \l_box_bottom_dim
      \l_box_bottom_new_dim
    \box_rotate_x:nnN \l_box_left_dim  \l_box_top_dim
      \l_box_left_new_dim
    \box_rotate_x:nnN \l_box_right_dim \l_box_bottom_dim
      \l_box_right_new_dim
  }
\cs_new_protected_nopar:Npn \box_rotate_quadrant_two:
  {
    \box_rotate_y:nnN \l_box_right_dim \l_box_bottom_dim
      \l_box_top_new_dim
    \box_rotate_y:nnN \l_box_left_dim  \l_box_top_dim
      \l_box_bottom_new_dim
    \box_rotate_x:nnN \l_box_right_dim  \l_box_top_dim
      \l_box_left_new_dim
    \box_rotate_x:nnN \l_box_left_dim   \l_box_bottom_dim
      \l_box_right_new_dim
  }
\cs_new_protected_nopar:Npn \box_rotate_quadrant_three:
  {
    \box_rotate_y:nnN \l_box_left_dim  \l_box_bottom_dim
      \l_box_top_new_dim
    \box_rotate_y:nnN \l_box_right_dim \l_box_top_dim
      \l_box_bottom_new_dim
    \box_rotate_x:nnN \l_box_right_dim \l_box_bottom_dim
      \l_box_left_new_dim
    \box_rotate_x:nnN \l_box_left_dim   \l_box_top_dim
      \l_box_right_new_dim
  }
\cs_new_protected_nopar:Npn \box_rotate_quadrant_four:
  {
    \box_rotate_y:nnN \l_box_left_dim  \l_box_top_dim
      \l_box_top_new_dim
    \box_rotate_y:nnN \l_box_right_dim \l_box_bottom_dim
      \l_box_bottom_new_dim
    \box_rotate_x:nnN \l_box_left_dim  \l_box_bottom_dim
      \l_box_left_new_dim
    \box_rotate_x:nnN \l_box_right_dim \l_box_top_dim
      \l_box_right_new_dim
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{variable}{\l_box_scale_x_fp, \l_box_scale_y_fp}
%   Scaling is potentially-different in the two axes.
%    \begin{macrocode}
\fp_new:N \l_box_scale_x_fp
\fp_new:N \l_box_scale_y_fp
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}{\box_resize:Nnn, \box_resize:cnn}
% \begin{macro}[aux]{\box_resize_aux:Nnn}
%   Resizing a box starts by working out the various dimensions of the
%   existing box.
%    \begin{macrocode}
\cs_new_protected:Npn \box_resize:Nnn #1#2#3
  {
    \hbox_set:Nn #1
      {
        \group_begin:
          \dim_set:Nn \l_box_top_dim    {  \box_ht:N #1 }
          \dim_set:Nn \l_box_bottom_dim { -\box_dp:N #1 }
          \dim_set:Nn \l_box_right_dim  {  \box_wd:N #1 }
          \dim_zero:N \l_box_left_dim
%    \end{macrocode}
%   The $x$-scaling and resulting box size is easy enough to work
%   out: the dimension is that given as |#2|, and the scale is simply the
%   new width divided by the old one.
%    \begin{macrocode}
          \fp_set_from_dim:Nn \l_box_scale_x_fp {#2}
          \fp_set_from_dim:Nn \l_box_tmp_fp { \l_box_right_dim }
          \fp_div:Nn \l_box_scale_x_fp { \l_box_tmp_fp }
%    \end{macrocode}
%   The $y$-scaling needs both the height and the depth of the current box.
%    \begin{macrocode}
          \fp_set_from_dim:Nn \l_box_scale_y_fp {#3}
          \fp_set_from_dim:Nn \l_box_tmp_fp
            { \l_box_top_dim - \l_box_bottom_dim }
          \fp_div:Nn \l_box_scale_y_fp { \l_box_tmp_fp }
%    \end{macrocode}
%   At this stage, check for trivial scaling. If both scalings are unity, then
%   the code does nothing. Otherwise, pass on to the auxiliary function to
%   find the new dimensions.
%    \begin{macrocode}
          \fp_compare:NNNTF \l_box_scale_x_fp = \c_one_fp
            {
              \fp_compare:NNNTF \l_box_scale_y_fp = \c_one_fp
                { \box_use:N #1 }
                { \box_resize_aux:Nnn #1 {#2} {#3} }
            }
            { \box_resize_aux:Nnn #1 {#2} {#3} }
        \group_end:
      }
  }
\cs_generate_variant:Nn \box_resize:Nnn { c }
%    \end{macrocode}
%   With at least one real scaling to do, the next phase is to find the new
%   edge co-ordinates. In the $x$~direction this is relatively easy: just
%   scale the right edge. This is done using the absolute value of the
%   scale so that the new edge is in the correct place. In the $y$~direction,
%   both dimensions have to be scaled, and this again needs the absolute
%   scale value. Once that is all done, the common resize/rescale code can
%   be employed.
%    \begin{macrocode}
\cs_new_protected:Npn \box_resize_aux:Nnn #1#2#3
  {
    \dim_compare:nNnTF {#2} > \c_zero_dim
      { \dim_set:Nn \l_box_right_new_dim {#2} }
      { \dim_set:Nn \l_box_right_new_dim { \c_zero_dim - ( #2 ) } }
    \dim_compare:nNnTF {#3} > \c_zero_dim
      {
        \dim_set:Nn \l_box_top_new_dim
          { \fp_use:N \l_box_scale_y_fp \l_box_top_dim }
        \dim_set:Nn \l_box_bottom_new_dim
          { \fp_use:N \l_box_scale_y_fp \l_box_bottom_dim }
      }
      {
        \dim_set:Nn \l_box_top_new_dim
          { - \fp_use:N \l_box_scale_y_fp \l_box_top_dim }
        \dim_set:Nn \l_box_bottom_new_dim
          { - \fp_use:N \l_box_scale_y_fp \l_box_bottom_dim }
      }
    \box_resize_common:N #1
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\box_resize_to_ht_plus_dp:Nn, \box_resize_to_ht_plus_dp:cn}
% \begin{macro}{\box_resize_to_wd:Nn, \box_resize_to_wd:cn}
%   Scaling to a total height or to a width is a simplified version of the main
%   resizing operation, with the scale simply copied between the two parts. The
%   internal auxiliary is called using the scaling value twice, as the sign for
%   both parts is needed (as this allows the same internal code to be used as
%   for the general case).
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \box_resize_to_ht_plus_dp:Nn #1#2
  {
    \hbox_set:Nn #1
      {
        \group_begin:
          \dim_set:Nn \l_box_top_dim    {  \box_ht:N #1 }
          \dim_set:Nn \l_box_bottom_dim { -\box_dp:N #1 }
          \dim_set:Nn \l_box_right_dim  {  \box_wd:N #1 }
          \dim_zero:N \l_box_left_dim
          \fp_set_from_dim:Nn \l_box_scale_y_fp {#2}
          \fp_set_from_dim:Nn \l_box_tmp_fp
            { \l_box_top_dim - \l_box_bottom_dim }
          \fp_div:Nn \l_box_scale_y_fp { \l_box_tmp_fp }
          \fp_set_eq:NN \l_box_scale_x_fp \l_box_scale_y_fp
          \fp_compare:NNNTF \l_box_scale_y_fp = \c_one_fp
            { \box_use:N #1 }
            { \box_resize_aux:Nnn #1 {#2} {#2} }
        \group_end:
      }
  }
\cs_generate_variant:Nn \box_resize_to_ht_plus_dp:Nn { c }
\cs_new_protected_nopar:Npn \box_resize_to_wd:Nn #1#2
  {
    \hbox_set:Nn #1
      {
        \group_begin:
          \dim_set:Nn \l_box_top_dim    {  \box_ht:N #1 }
          \dim_set:Nn \l_box_bottom_dim { -\box_dp:N #1 }
          \dim_set:Nn \l_box_right_dim  {  \box_wd:N #1 }
          \dim_zero:N \l_box_left_dim
          \fp_set_from_dim:Nn \l_box_scale_x_fp {#2}
          \fp_set_from_dim:Nn \l_box_tmp_fp { \l_box_right_dim }
          \fp_div:Nn \l_box_scale_x_fp { \l_box_tmp_fp }
          \fp_set_eq:NN \l_box_scale_y_fp \l_box_scale_x_fp
          \fp_compare:NNNTF \l_box_scale_x_fp = \c_one_fp
            { \box_use:N #1 }
            { \box_resize_aux:Nnn #1 {#2} {#2} }
        \group_end:
      }
  }
\cs_generate_variant:Nn \box_resize_to_wd:Nn { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\box_scale:Nnn, \box_scale:cnn}
% \begin{macro}[aux]{\box_scale_aux:Nnn}
%   When scaling a box, setting the scaling itself is easy enough. The
%   new dimensions are also relatively easy to find, allowing only for
%   the need to keep them positive in all cases. Once that is done then
%   after a check for the trivial scaling a hand-off can be made to the
%   common code. The dimension scaling operations are carried out using
%   the \TeX{} mechanism as it avoids needing to use \texttt{fp}
%   operations.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \box_scale:Nnn #1#2#3
  {
    \hbox_set:Nn #1
      {
        \group_begin:
          \fp_set:Nn \l_box_scale_x_fp {#2}
          \fp_set:Nn \l_box_scale_y_fp {#3}
          \dim_set:Nn \l_box_top_dim    {  \box_ht:N #1 }
          \dim_set:Nn \l_box_bottom_dim { -\box_dp:N #1 }
          \dim_set:Nn \l_box_right_dim  {  \box_wd:N #1 }
          \dim_zero:N \l_box_left_dim
          \fp_compare:NNNTF \l_box_scale_x_fp = \c_one_fp
            {
              \fp_compare:NNNTF \l_box_scale_y_fp = \c_one_fp
                { \box_use:N #1 }
                { \box_scale_aux:Nnn #1 {#2} {#3} }
            }
            { \box_scale_aux:Nnn #1 {#2} {#3} }
        \group_end:
      }
  }
\cs_generate_variant:Nn \box_scale:Nnn { c }
\cs_new_protected_nopar:Npn \box_scale_aux:Nnn #1#2#3
  {
    \fp_compare:NNNTF \l_box_scale_y_fp > \c_zero_fp
      {
        \dim_set:Nn \l_box_top_new_dim    { #3 \l_box_top_dim }
        \dim_set:Nn \l_box_bottom_new_dim { #3 \l_box_bottom_dim }
      }
      {
        \dim_set:Nn  \l_box_top_new_dim    { -#3 \l_box_bottom_dim }
        \dim_set:Nn  \l_box_bottom_new_dim { -#3 \l_box_top_dim }
      }
    \fp_compare:NNNTF \l_box_scale_x_fp > \c_zero_fp
      { \l_box_right_new_dim #2 \l_box_right_dim }
      { \l_box_right_new_dim -#2 \l_box_right_dim }
    \box_resize_common:N #1
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}[int]{\box_resize_common:N}
%   The main resize function places in input into a box which will start
%   of with zero width, and includes the handles for engine rescaling.
%    \begin{macrocode}
\cs_new_protected_nopar:Npn \box_resize_common:N #1
  {
    \hbox_set:Nn \l_box_tmp_box
      {
        \driver_box_scale_begin:
        \hbox_overlap_right:n { \box_use:N #1 }
        \driver_box_scale_end:
      }
%    \end{macrocode}
%   The new height and depth can be applied directly.
%    \begin{macrocode}
    \box_set_ht:Nn \l_box_tmp_box { \l_box_top_new_dim }
    \box_set_dp:Nn \l_box_tmp_box { \l_box_bottom_new_dim }
%    \end{macrocode}
%   Things are not quite as obvious for the width, as the reference point
%   needs to remain unchanged. For positive scaling factors resizing the
%   box is all that is needed. However, for case of a negative scaling
%   the material must be shifted such that the reference point ends up in
%   the right place.
%    \begin{macrocode}
    \fp_compare:NNNTF \l_box_scale_x_fp < \c_zero_fp
      {
        \hbox_to_wd:nn { \l_box_right_new_dim }
          {
            \tex_kern:D \l_box_right_new_dim
            \box_use:N \l_box_tmp_box
            \tex_hss:D
          }
      }
      {
        \box_set_wd:Nn \l_box_tmp_box { \l_box_right_new_dim }
        \box_use:N \l_box_tmp_box
      }
  }
%    \end{macrocode}
%\end{macro}
%
%    \begin{macrocode}
%</initex|package>
%    \end{macrocode}
%
% \end{implementation}
%
% \PrintIndex