% \iffalse meta-comment
%
% File: ltx-talk-overlay.dtx Copyright (C) 2024-2026 Joseph Wright
%
% It may be distributed and/or modified under the conditions of the
% LaTeX Project Public License (LPPL), either version 1.3c of this
% license or (at your option) any later version.  The latest version
% of this license is in the file
%
%    https://www.latex-project.org/lppl.txt
%
% This file is part of the "ltx-talk 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
%
%    https://github.com/josephwright/ltx-talk
%
% for those people who are interested.
%
% -----------------------------------------------------------------------
%
%<*driver>
\documentclass{l3doc}
% Additional commands needed in this source
\NewDocumentCommand\email{m}{\href{mailto:#1}{\nolinkurl{#1}}}
\begin{document}
  \DocInput{\jobname.dtx}
\end{document}
%</driver>
% \fi
%
% ^^A As we are dealing with a class, this has to be done manually
% \def\filedate{2026-04-14}
% \def\fileversion{v0.4.11}
%
% \title{^^A
%   \pkg{ltx-talk-overlay} -- Overlays^^A
%   \thanks{This file describes \fileversion,
%     last revised \filedate.}^^A
% }
%
% \author{^^A
%  Joseph Wright^^A
%  \thanks{^^A
%    E-mail: \email{joseph@texdev.net}^^A
%   }^^A
% }
%
% \date{Released \filedate}
%
% \maketitle
%
% \begin{documentation}
%
% \end{documentation}
%
% \begin{implementation}
%
% \section{\pkg{ltx-talk-overlay} implementation}
%
% Start the \pkg{DocStrip} guards.
%    \begin{macrocode}
%<*class>
%    \end{macrocode}
%
% Identify the internal prefix.
%    \begin{macrocode}
%<@@=talk>
%    \end{macrocode}
%
% \subsection{Utilities}
%
% \begin{macro}[TF]{\@@_if_overlay:n, \@@_if_overlay:V}
% \begin{macro}{\@@_overlay_arg:n}
%    \begin{macrocode}
\prg_new_protected_conditional:Npnn \@@_if_overlay:n #1 { T , F , TF }
  {
    \@@_decode_parse:n {#1}
    \bool_if:NTF \l_@@_decode_overlays_bool
      \prg_return_true:
      \prg_return_false:
  }
\prg_generate_conditional_variant:Nnn \@@_if_overlay:n { V } { T , F , TF }
%    \end{macrocode}
%   A macro processor variant of the check that always results in an |N|-type
%   bool.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_overlay_arg:n #1
  {
    \@@_if_overlay:nTF {#1}
      { \cs_set:Npn \ProcessedArgument { \c_true_bool } }
      { \cs_set:Npn \ProcessedArgument { \c_false_bool } }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{variable}{\l_@@_shuffle_skip}
%   For tracking.
%    \begin{macrocode}
\skip_new:N \l_@@_shuffle_skip
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}{\@@_shuffle_skip:n}
%   As opacity uses whatsits at present, we need to make sure that any spaces
%   come \emph{after} them. This is done by \enquote{shuffling} the last skip
%   past the opacity.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_shuffle_skip:n #1
  {
    \skip_set_eq:NN \l_@@_shuffle_skip \tex_lastskip:D
    \bool_lazy_and:nnTF
      { ! \skip_if_eq_p:nn \l_@@_shuffle_skip { 0pt } }
      {
        \bool_lazy_or_p:nn
          { \mode_if_horizontal_p: }
          { \mode_if_vertical_p: }
      }
      {
        \tex_unskip:D
        #1
        \mode_if_horizontal:TF
          { \skip_horizontal:n }
          { \skip_vertical:n }
            \l_@@_shuffle_skip
      }
      {#1}
  }
%    \end{macrocode}
% \end{macro}
%
% \subsection{Opacity utilities}
%
% Currently, opacity is applies using whatsits at a low level. That means
% that to preserve spacing, we need to insert no-op versions in various
% places. To do that and get correct overlays, we need to track the current
% opacity. At present, this seems very \cls{ltx-talk}-specific, so
% is handled here with a few auxiliaries.
%
% \begin{macro}{\@@_opacity_begin:n, \@@_opacity_end:}
%   Simply tracking wrappers.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_opacity_begin:n #1
  { \@@_shuffle_skip:n { \opacity_begin:n {#1} } }
\cs_new_protected:Npn \@@_opacity_end:
  { \@@_shuffle_skip:n { \opacity_end: } }
%    \end{macrocode}
% \end{macro}
%
% \subsection{Action commands and environments}
%
% Commands that can be used as actions all have a common form (with one
% exception). The common internal structure is used to enable them to be used
% as actions by looking for the name \cs[no-index]{@@_action_\meta{name}:N}.
%
% \begin{macro}{\@@_action_alert:N}
%   At present a color selection.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_action_alert:N #1
  {
    \bool_if:NTF #1
      { \color_select:n { alert } }
      { \color_select:n { . } }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}
%   {
%     \@@_action_invisible:N     ,
%     \@@_action_invisible_end:N ,
%     \@@_action_visible:N       ,
%     \@@_action_visible_end:N
%   }
%   Simply (un)hide unconditionally, overwriting any previous opacity.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_action_invisible:N #1
  {
    \bool_if:NTF #1
      { \@@_opacity_begin:n { 0 } }
      { \@@_opacity_begin:n { 1 } }
  }
\cs_new_protected:Npn \@@_action_invisible_end:N #1
  { \@@_opacity_end: }
\cs_new_protected:Npn \@@_action_visible:N #1
  {
    \bool_if:NTF #1
      { \@@_opacity_begin:n { 1 } }
      { \@@_opacity_begin:n { 0 } }
  }
\cs_new_protected:Npn \@@_action_visible_end:N #1
  { \@@_opacity_end: }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_action_only:N, \@@_action_only_end:N}
%   Here, we simply throw away the content we do not want: this is done by
%   typesetting in a disposable box.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_action_only:N #1
  {
    \bool_if:NF #1
      { \vbox_set:Nw \l_@@_tmp_box }
  }
\cs_new_protected:Npn \@@_action_only_end:N #1
  {
    \bool_if:NF #1
      { \vbox_set_end: }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{variable}{\l_@@_uncover_hidden_fp}
%   Currently just an on-off, but that will change.
%    \begin{macrocode}
\NewTemplateType { hidden } { 0 }
\DeclareTemplateInterface { hidden } { talk } { 0 }
  { opacity : real = 0 }
\DeclareTemplateCode { hidden } { talk } { 0 }
  { opacity = \l_@@_uncover_hidden_fp }
  { \@@_opacity_begin:n { \l_@@_uncover_hidden_fp } }
\DeclareInstance { hidden } { std } { talk } { }
%    \end{macrocode}
% \end{variable}
% \begin{macro}{\@@_action_uncover:N, \@@_action_uncover_end:N}
%   Use the template: we may need to extend that to deal with the
%   end-of-template case later.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_action_uncover:N #1
  {
    \bool_if:NTF #1
      { \@@_opacity_begin:n { 1 } }
      { \UseInstance { hidden } { std } }
  }
\cs_new_protected:Npn \@@_action_uncover_end:N #1
  { \@@_opacity_end: }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\invisible, \uncover, \visible}
%   All generated automatically using the above implementations.
%    \begin{macrocode}
\clist_map_inline:nn { invisible , uncover , visible }
  {
    \ExpandArgs { cne } \NewDocumentCommand {#1}
      { > { \@@_overlay_arg:n } D <> { all } +m }
      {
        \exp_not:c { @@_action_ #1 :N } ##1
        ##2
        \exp_not:c { @@_action_ #1 _end:N } ##1
      }
%    \end{macrocode}
% \end{macro}
% \begin{environment}{invisibleenv, uncoverenv, visibleenv}
%   And the environment versions.
%    \begin{macrocode}
    \ExpandArgs { nnee } \NewDocumentEnvironment { #1 env }
      { > { \@@_overlay_arg:n } D <> { all } }
      { \exp_not:c { @@_action_ #1 :N } ##1 }
      { \exp_not:c { @@_action_ #1 _end:N } ##1 }
  }
%    \end{macrocode}
% \end{environment}
%
% \begin{macro}{\alert}
%   The \cs{alert} command requires a group to contain color, so is done
%   separately even though it still uses basically the same mechanism.
%    \begin{macrocode}
\NewDocumentCommand \alert { > { \@@_overlay_arg:n } D <> { all } +m }
  {
    \group_begin:
      \@@_action_alert:N #1
        #2
     \group_end:
  }
%    \end{macrocode}
% \end{macro}
% \begin{environment}{alertenv}
%   As does the environment.
%    \begin{macrocode}
\NewDocumentEnvironment { alertenv } { > { \@@_overlay_arg:n } D <> { all } }
  { \@@_action_alert:N #1 }
  { }
%    \end{macrocode}
% \end{environment}
% \begin{macro}{\only}
%   This code needs to be done manually as for the command version the content
%   must be entirely discarded. That can't work for the environment version,
%   which has to deal with for example single items in a list (and so cannot
%   be collected up verbatim and must use a box).
%    \begin{macrocode}
\NewDocumentCommand \only { D <> { all } +m }
  {
    \@@_if_overlay:nT {#1}
      {#2}
  }
%    \end{macrocode}
% \end{macro}
% \begin{environment}{onlyenv}
%   The environment version could be done above, but it is clearer to keep
%   this code entirely separate from the rest.
%    \begin{macrocode}
\NewDocumentEnvironment { onlyenv } { > { \@@_overlay_arg:n } D <> { all } }
  { \@@_action_only:N #1 }
  { \@@_action_only_end:N #1 }
%    \end{macrocode}
% \end{environment}
%
% \begin{variable}
%   {
%     \l_@@_saved_overlays_bool ,
%     \l_@@_saved_action_str    ,
%     \l_@@_saved_actions_bool
%   }
%    \begin{macrocode}
\bool_new:N \l_@@_saved_overlays_bool
\str_new:N \l_@@_saved_action_str
\bool_new:N \l_@@_saved_actions_bool
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}
%   {\l_@@_overlay_all_bool}
%    \begin{macrocode}
\bool_new:N \l_@@_overlay_all_bool
%    \end{macrocode}
% \end{variable}
% \begin{macro}{\action}
% \begin{environment}{actionenv}
% \begin{macro}{\@@_action_begin:n}
% \begin{macro}{\@@_action_begin:w}
% \begin{macro}{\@@_action_begin_auxi:n, \@@_action_begin_auxii:n}
% \begin{macro}{\@@_action_end:}
%   As we need data on not just overlays but also actions at the end of the
%   environment, this has to be done manually. To allow working with
%   environments but also items, the code needs to save data for the end
%   function. The group is needed for cases where we are not in a \LaTeX{}
%   environment group. When an \cs{onslide}/\cs{pause} is active, it takes
%   priority: sorted by applying up-front. Actions can be skipped entirely
%   if the overlay spec is simply \texttt{all}, as there will never be
%   any spacing issues, \foreign{etc}.
%    \begin{macrocode}
\NewDocumentCommand \action { d <> +m }
  {
    \group_begin:
      \@@_action_begin:n {#1}
      #2
      \@@_action_end:
    \group_end:
  }
\NewDocumentEnvironment { actionenv } { d <> }
  { \@@_action_begin:n {#1} }
  { \@@_action_end: }
\cs_new_protected:Npn \@@_action_begin:n #1
  {
    \group_begin:
      \tl_if_novalue:nTF {#1}
        {
          \exp_after:wN \@@_action_begin:w
            \l_@@_action_spec_str < all > \q_stop
        }
        { \@@_action_begin_auxi:n {#1} }
  }
\cs_new_protected:Npn \@@_action_begin:w #1 < #2 > #3 \q_stop
  { \@@_action_begin_auxi:n {#2} }
\cs_new_protected:Npn \@@_action_begin_auxi:n #1
  {
    \str_if_eq:nnTF {#1} { all }
      { \bool_set_true:N \l_@@_overlay_all_bool }
      {
        \bool_set_false:N \l_@@_overlay_all_bool
        \@@_action_begin_auxii:n {#1}
      }
  }
\cs_new_protected:Npn \@@_action_begin_auxii:n #1
  {
      \@@_decode_parse:n {#1}
      \bool_set_eq:NN \l_@@_saved_overlays_bool
        \l_@@_decode_overlays_bool
      \str_set_eq:NN \l_@@_saved_action_str
        \l_@@_decode_action_str
      \bool_set_eq:NN \l_@@_saved_actions_bool
        \l_@@_decode_actions_bool
      \tl_if_empty:NTF \g_@@_onslide_tl
        {
          \bool_if:NTF \l_@@_decode_overlays_bool
            {
              \cs_if_exist_use:cF
                { @@_action_ \l_@@_decode_action_str :N }
                  { \use_none:n }
                    \l_@@_decode_actions_bool
            }
            { \UseInstance { hidden } { std } }
        }
        { \@@_action_invisible:N \c_true_bool }
  }
\cs_new_protected:Npn \@@_action_end:
  {
      \bool_if:NF \l_@@_overlay_all_bool
        {
          \tl_if_empty:NTF \g_@@_onslide_tl
            {
              \bool_if:NTF \l_@@_saved_overlays_bool
                {
                  \cs_if_exist_use:cF
                    { @@_action_ \l_@@_saved_action_str _end:N }
                    { \use_none:n }
                      \l_@@_saved_actions_bool
                }
                { \@@_opacity_end: }
            }
            { \@@_action_invisible_end:N \c_true_bool }
        }
    \group_end:
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{environment}
% \end{macro}
%
% \subsection{Non-action commands and environments}
%
% This section contains commands and environments that do \emph{not}
% need to be made available as actions.
%
% \begin{macro}{\alt}
%   Simple wrappers around the internal switch.
%    \begin{macrocode}
\NewDocumentCommand \alt { D <> { all } +m +m }
  {
    \@@_if_overlay:nTF {#1}
      {#2}
      {#3}
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\onslide}
% \begin{macro}{\@@_onslide:n}
%   Simply make transparent: this is done without grouping so we can work
%   for example in tabular cells.
%    \begin{macrocode}
\NewDocumentCommand \onslide { D <> { all } }
  {
    \@@_onslide:n {#1}
    \ignorespaces
  }
\cs_new_protected:Npn \@@_onslide:n #1
  {
    \tl_use:N \g_@@_onslide_tl
    \tl_gclear:N \g_@@_onslide_tl
    \@@_if_overlay:nF {#1}
      {
        \@@_opacity_begin:n { 0 }
        \tl_gput_right:Nn \g_@@_onslide_tl
          { \@@_opacity_end: }
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \begin{variable}{\g_@@_onslide_tl}
%    \begin{macrocode}
\tl_new:N \g_@@_onslide_tl
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}{\temporal}
%   A tricky one: to separate the not-on-current-slide cases, the flag to
%   continue is used.
%    \begin{macrocode}
\NewDocumentCommand \temporal { D <> { all } +m +m +m }
  {
    \@@_if_overlay:nTF {#1}
      {#3}
      {
        \bool_if:NTF \g_@@_slide_continue_bool
          {#4}
          {#2}
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\pause}
%   A thin wrapper.
%    \begin{macrocode}
\NewDocumentCommand \pause { o }
  {
    \legacy_if:nF { measuring@ }
      {
        \IfNoValueTF {#1}
          { \int_gincr:N \g_@@_pauses_int }
          { \int_gset:Nn \g_@@_pauses_int {#1} }
        \exp_args:Ne \@@_onslide:n { \int_eval:n { \g_@@_pauses_int + 1 } - }
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \subsection{Fixed-size areas}
%
% \begin{macro}{\@@_overprint_begin:n}
%   A common auxiliary for overprinting, which starts off much the same for
%   both \env{overlayarea} and \env{overprint}.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_overprint_begin:n #1
  {
    \par
    \vbox_set_to_wd:Nnw \l_@@_tmp_box {#1}
      \raggedright
      \ignorespaces
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{environment}{overlayarea}
%   An initial approach: quite similar to a column.
%    \begin{macrocode}
\NewDocumentEnvironment { overlayarea } { m m }
  { \@@_overprint_begin:n {#1} }
  {
    \vbox_set_end:
    \vbox_to_ht:nn {#2}
      {
        \box_use_drop:N \l_@@_tmp_box
        \vfil
      }
    \par
  }
%    \end{macrocode}
% \end{environment}
%
% \begin{variable}{\l_@@_overprint_int}
%   Track the overprints on a slide: as the slide forms a group, we do not need
%   to worry about resetting.
%    \begin{macrocode}
\int_new:N \l_@@_overprint_int
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}[EXP]{\@@_frame_overprint:}
%   To refer to the current overprint environment within the document:
%   needed in the \texttt{.aux} so avoids using non-letters.
%    \begin{macrocode}
\cs_new:Npn \@@_frame_overprint:
  {
    \int_to_Roman:n \g_@@_frame_int
    \int_to_roman:n \l_@@_overprint_int
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{environment}{overprint}
% \begin{macro}{\@@_overprint_save_ht:}
% \begin{macro}{\@@_overprint_check_ht:n}
%   For overprinting, in contrast to \cls{beamer} we use a two-pass approach
%   to save the size at the end of the run: this means you can use \cs{only}
%   for example in overprinting.
%    \begin{macrocode}
\NewDocumentEnvironment { overprint } { O { \textwidth } }
  { \@@_overprint_begin:n {#1} }
  {
    \vbox_set_end:
    \int_incr:N \l_@@_overprint_int
    \@@_overprint_save_ht:
    \cs_if_exist:cTF 
      { overprint@ \@@_frame_overprint: }
      {
        \dim_compare:vNnTF
          { overprint@ \@@_frame_overprint: }
            > { \box_ht:N \l_@@_tmp_box }
          {
            \vbox_to_ht:vn
              { overprint@ \@@_frame_overprint: }
              {
                \box_use_drop:N \l_@@_tmp_box
                \vfil
              }
          }
          { \box_use_drop:N \l_@@_tmp_box }
      }
      { \box_use_drop:N \l_@@_tmp_box }
    \par
  }
%    \end{macrocode}
%   As there is no clear end-point for overprinting, we need to be careful to
%   keep the current width separate from the saved one. The rest is then about
%   saving to the \texttt{.aux} file and helping out the user. 
%    \begin{macrocode}
\cs_new_protected:Npn \@@_overprint_save_ht:
  {
    \tl_if_exist:cF { g_@@_overprint_ \@@_frame_overprint: _tl }
      {
        \tl_new:c { g_@@_overprint_ \@@_frame_overprint: _tl }
        \tl_gset:cn { g_@@_overprint_ \@@_frame_overprint: _tl }
          { 0pt }
      }
    \tl_gset:ce { g_@@_overprint_ \@@_frame_overprint: _tl }
      {
        \dim_max:vn { g_@@_overprint_ \@@_frame_overprint: _tl }
          { \box_ht:N \l_@@_tmp_box }
      }
    \legacy_if:nT { @filesw }
      {
        \iow_now:Ne \@auxout
          {
            \gdef \exp_not:c { overprint@ \@@_frame_overprint: }
              {
                \exp_not:v { g_@@_overprint_ \@@_frame_overprint: _tl }
              }
          }
      }
    \hook_gput_code:nne { enddocument / afterlastpage } { talk }
       { \@@_overprint_check_ht:n { \@@_frame_overprint: } }
  }
\cs_new_protected:Npn \@@_overprint_check_ht:n #1
  {
    \bool_lazy_and:nnF
      { \exp_not:N \cs_if_exist_p:c { overprint@ #1 } }
      {
        \dim_compare_p:vNv { overprint@ #1 } = { g_@@_overprint_ #1 _tl }
      }
      {
        \msg_warning:nn { talk } { overprint-ht }
        \cs_gset_protected:Npn \@@_overprint_check_ht:n ##1 { }
      }
  }
\msg_new:nnn { talk } { overprint-ht }
  {
    Overprint~area~height~has~changed:\\
    rerun~LaTeX.
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{environment}
%
% \subsection{Adding overlays to existing commands}
%
% \begin{macro}
%  {
%    \textbf ,
%    \textit ,
%    \textmd ,
%    \textnormal ,
%    \textrm ,
%    \textsc ,
%    \textsf ,
%    \textsl ,
%    \texttt ,
%    \textup ,
%    \emph
%  }
% \begin{macro}
%  {
%    \stdtextbf ,
%    \stdtextit ,
%    \stdtextmd ,
%    \stdtextnormal ,
%    \stdtextrm ,
%    \stdtextsc ,
%    \stdtextsf ,
%    \stdtextsl ,
%    \stdtexttt ,
%    \stdtextup ,
%    \stdemph
%  }

% \begin{macro}{\@@_textcmd_eqiv:n}
%   Make the standard text commands overlay-aware. To keep the spacing
%   unchanged when the command is not active, we use the same approach as
%   the kernel does for inserting the right grouping.
%    \begin{macrocode}
\tl_map_inline:nn
  {
    \textbf
    \textit
    \textmd
    \textnormal
    \textrm
    \textsc
    \textsf
    \textsl
    \texttt
    \textup
    \emph
  }
  {
    \ExpandArgs { c } \NewCommandCopy { std \cs_to_str:N #1 } #1
    \ExpandArgs { Nne } \RenewDocumentCommand #1
      { D <> { all } +m }
      {
        \exp_not:N \@@_if_overlay:nTF {##1}
          { \exp_not:c { std \cs_to_str:N #1 } }
          { \exp_not:N \@@_textcmd_eqiv:n }
            {##2}
      }
  }
\cs_new_protected:Npn \@@_textcmd_eqiv:n #1
  {
    \mode_if_math:TF
      { { \mbox {#1} } }
      {
        \mode_leave_vertical:
        {#1}
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\includegraphics}
% \begin{macro}{\stdincludegraphics}
%   Just wrap up the args and forward if appropriate. The star is |#1| here as
%   that matches the documented behavior of starred commands generally.
%    \begin{macrocode}
\RequirePackage { graphicx }
\NewCommandCopy \stdincludegraphics \includegraphics
\RenewDocumentCommand \includegraphics { s D <> { all } o o m }
  {
    \@@_if_overlay:nT {#2}
      {
        \use:e
          {
            \exp_not:N \stdincludegraphics
              \IfBooleanT #1 { * }
              \IfNoValueF {#3} { [ \exp_not:n { {#3} } ] }
              \IfNoValueF {#4} { [ \exp_not:n { {#4} } ] }
          }
            {#5}
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\label}
% \begin{macro}{\@@_label:n}
%   Here, we can't wrap the existing command up as we need the space hack, so
%   it has to be declared from scratch. There is also a non-standard overlay
%   default. At present, no special tricks as seen in \pkg{beamer}.   
%    \begin{macrocode}
\RenewDocumentCommand \label { D <> { 1 } m }
  {
    \@bsphack
    \@@_if_overlay:nT {#1}
      { \@@_label:n {#2} }
    \@esphack
  }
\cs_new_protected:Npn \@@_label:n #1
  {
    \begingroup
      \UseHookWithArguments { label } { 1 } {#1}
      \protected@write \@auxout { }
        {
          \string \newlabel {#1}
            { 
              { \@currentlabel }
              { \thepage }
              { \@currentlabelname }
              { \@currentHref }
              { \@kernel@reserved@label@data }
            }
        }
    \endgroup
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \begin{macro}{\std@label@in@display, \std@label@in@display}
%   We also need to cover \cs{label@in@display}: a bit more awkward as this is
%   a document command but not set up as such in \pkg{amsmath}. For the present,
%   cross our fingers on this!
%    \begin{macrocode}
\cs_new_eq:NN \std@label@in@display \label@in@display
\RenewDocumentCommand \label@in@display { D <> { 1 } m }
  {
    \@@_if_overlay:nT {#1}
      { \std@label@in@display {#2} }
  }
%    \end{macrocode}
% \end{macro}
%
% \subsection{Overlay patching of third-party environments}
%
% To allow opacity to apply to the entirety of constructed graphics, we need to
% apply a transparency group around the whole thing. We need to do that by saving
% the input in an Xform object, which then allows the group to be applied.
%
% \begin{variable}{\g_@@_opacity_group_int}
%   Each use needs an Xform object, which at present we have to track as
%   there are no anonymous ones.
%    \begin{macrocode}
\int_new:N \g_@@_opacity_group_int
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}{\@@_opacity_group_begin:, \@@_opacity_group_end:}
%   A general mechanism to collect up an environment in a box, which we can
%   then use as the source for an Xform object.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_opacity_group_begin:
  { \begin { lrbox } \l_@@_tmp_box }
\cs_new_protected:Npn \@@_opacity_group_end:
  {
    \end { lrbox }
    \mode_leave_vertical:
    \int_gincr:N \g_@@_opacity_group_int
    \exp_args:Ne \pdfxform_new:nnn
      { talk.opacity \int_use:N \g_@@_opacity_group_int }
      { /Group << /S /Transparency /K ~ false /I ~ false >> }
      { \usebox \l_@@_tmp_box }
    \exp_args:Ne \pdfxform_use:n
      { talk.opacity \int_use:N \g_@@_opacity_group_int }
  }
%    \end{macrocode}
% \end{macro}
%
% At present, we only add code onto the main top-level functions. This is only
% applied in Lua\TeX{} due to restrictions on Xform structures in \pdfTeX{}.
% Here, we apply the collection code to the commands not the environment
% forms to deal with \enquote{direct} usage.
%    \begin{macrocode}
\sys_if_engine_luatex:T
  {
    \AddToHook { cmd / pspicture / before } { \@@_opacity_group_begin: }
    \AddToHook { cmd / endpspicture / after } { \@@_opacity_group_end: }
  }
%    \end{macrocode}
%
%    \begin{macrocode}
%</class>
%    \end{macrocode}
%
% \end{implementation}
%
% \PrintIndex