% We need to know the vertical position on the page where the figures
% are located. Because this position can't be known until the page is
% fully composed ("shipout"), the value is only accurate after the
% second run, just like the \ref command. The value from the previous
% run is available in the .aux file.
\RequirePackage{zref-savepos}

% The zref package also provides the page number. It gives the
% absolute page number, not the logical page number. Things like the
% table of contents and preface are part of the count. This value is
% available from abspage. This number is zero-based, so that the
% "first" page is numbered as zero. Just as positions within a page
% are not really known until a page is shipped out, the page number
% isn't known either. Generally, if you're near the middle or the end
% of the page, then \thepage or abspage is correct, but it is often
% wrong near the top of the page. The only way around this seems to be
% to place a label on the page, in a manner similar to what is done
% for positions within the page.
\RequirePackage{zref-thepage}
\RequirePackage{zref-abspage}
\RequirePackage{zref-user}
\RequirePackage{zref-pagelayout}
\RequirePackage{xsim}
\RequirePackage{tikz}

% This is needed to help with the process of replacing the body of
% the environment and (optionally) copying it to an external file.
\RequirePackage{verbatim}


\ExplSyntaxOn

% The fig.aux file is open throughout the run.
\iow_new:N \g_figurefile_iow
\iow_open:Nn \g_figurefile_iow { \jobname.fig.aux }

% The inner and outer margin widths
\dim_new:N \l_figput_innermargin_dim
\dim_new:N \l_figput_outermargin_dim

% By default, set these margins to have zero width. This has no effect
% on how the text is laid out by latex. It's used on the browser end
% to set the origin for drawing. In turn, that affects where the
% figures end up when the tikz is loaded.
%
% BUG: There should be a way (?) to determine where the left margin of
% the text is from latex instead of inputing your best approximation.
% At the same time, the user might want to adjust this regardless.
\dim_set:Nn \l_figput_innermargin_dim { 0 pt }
\dim_set:Nn \l_figput_outermargin_dim { 0 pt }

% Two little commands to set these values.
\NewDocumentCommand\SetInnerMargin { m }
{
  \dim_set:Nn \l_figput_innermargin_dim { #1 }
}

\NewDocumentCommand\SetOuterMargin { m }
{
  \dim_set:Nn \l_figput_outermargin_dim { #1 }
}

% Use this to turn off any of the optional "skip" arguments to figput.
% If you call \NeverSkip, then these "skip" arguments are ignored.
\bool_new:N \l_figput_skipoff_bool
\bool_set_false:N \l_figput_skipoff_bool

\NewDocumentCommand\NeverSkip{} {
  \bool_set_true:N \l_figput_skipoff_bool
}

\NewDocumentCommand\AllowSkip{} {
  \bool_set_false:N \l_figput_skipoff_bool
}

% Use this to request that a specific file be loaded by the browser.
% This simply writes a line to the .aux file.
\NewDocumentCommand\LoadFigureCode{ m }
{
  \group_begin:
  
  \iow_now:Nx \g_figurefile_iow { load~ #1 }
  
  \group_end:
}


% The variables used below:
%
% Manditory name for the figure, as given by the user. Using string,
% although maybe token list would be better?
\str_new:N \l_figput_figname_str

% The manditory height given by the user. This is a dim, as opposed to
% merely a floating-point value. 
\dim_new:N \l_figput_height_dim

% The optional height adjustments given by the user. Again, dims.
\dim_new:N \l_figput_htAbove_dim
\dim_new:N \l_figput_htBelow_dim

% Scratch values used for calculation of figure position.
\tl_new:N \l_figput_tempa_tl
\tl_new:N \l_figput_tempb_tl
\tl_new:N \l_figput_tempc_tl

% And several booleans.
% BUG: In theory, it would be possible to get rid of nostatic.
% If the figure height is set to zero, then that should implicitly
% imply nostatic.
\bool_new:N \l_figput_hasintht_bool
\bool_new:N \l_figput_nostatic_bool
\bool_new:N \l_figput_done_bool
\bool_new:N \l_figput_skip_bool

% This is for the figures file: the position of the figure on the page.
\dim_new:N \l_figput_pagepos_dim

% BUG: I don't understand the whole concept of these variants of
% functions. Somehow it deals with various cases of whether arguments 
% are expanded or not. There's another variant defintion below as well.
\cs_generate_variant:Nn \file_if_exist:nTF {V}
\cs_generate_variant:Nn \file_input:n {V}



% This command works in (almost) exactly the same way as the figput 
% environment. The differenence is that, as a command instead of
% an environment, there is no body to deal with. Most of the comments
% have been removed. See the definition of the environment for *much*
% more detailed comments.
%
% BUG: possible to define a function and combine these?
\NewDocumentCommand\FigPut { > { \SplitArgument { 1 } { , } } m!o }
{
  \group_begin:

  % The manditory arguments come in in #1
  \figput_manargs #1

  % Now the optional arguments.
  \bool_set_false:N \l_figput_hasintht_bool
  \bool_set_false:N \l_figput_nostatic_bool
  \bool_set_false:N \l_figput_done_bool
  \bool_set_false:N \l_figput_skip_bool
  
  \tl_if_blank:nTF { #2 } {} {
    \clist_set:Nn \l_figput_optclist_clist { #2 }
    \figput_optargs { } 
  }

  % We have everything needed to know what to do.
  \str_set:Nx \l_figput_filespec_str { 
    \use:c { zref@extractdefault } { \l_figput_figname_str -pageno} { abspage } { 0 }
  }

  % The inner and outer margins.
  \str_put_right:Nx \l_figput_filespec_str { ~ \dim_to_decimal_in_bp:n \l_figput_innermargin_dim }
  \str_put_right:Nx \l_figput_filespec_str { ~ \dim_to_decimal_in_bp:n \l_figput_outermargin_dim }

  % And the text width.
  \str_put_right:Nx \l_figput_filespec_str { ~ \dim_to_decimal_in_bp:n \textwidth }

  % Massive fooling around to do arithmetic on page position.
  \tl_set:Nn \l_figput_tempa_tl { \zposy { \l_figput_figname_str } }
  % Convert the figure height to sp.
  \tl_set:Nn \l_figput_tempb_tl { \dim_to_decimal_in_sp:n{ \l_figput_height_dim } }
  % Add 
  \tl_set:Nn \l_figput_tempc_tl { \int_eval:n { \l_figput_tempa_tl + \l_figput_tempb_tl } }
  % Convert to a dim
  \dim_set:Nn \l_figput_pagepos_dim { \l_figput_tempc_tl sp }
  % And (whew!) write it out, in bp.
  \str_put_right:Nx \l_figput_filespec_str { ~ \dim_to_decimal_in_bp:n \l_figput_pagepos_dim }

  % The latex height of the figure (in bp)
  \str_put_right:Nx \l_figput_filespec_str { ~ \dim_to_decimal_in_bp:n \l_figput_height_dim }

  % The interactive height paddings, which are zero by default.
  \bool_if:NTF \l_figput_hasintht_bool
  { 
    \str_put_right:Nx \l_figput_filespec_str { ~ \dim_to_decimal_in_bp:n \l_figput_htAbove_dim }
    \str_put_right:Nx \l_figput_filespec_str { ~ \dim_to_decimal_in_bp:n \l_figput_htBelow_dim }
  }
  {
    \str_put_right:Nx \l_figput_filespec_str { ~0~0 }
  }

  % The figure name -- used for the JS function to call.
  \str_put_right:Nx \l_figput_filespec_str { ~ \l_figput_figname_str }

  % And a boolean for whether the tikz is 'done.'
  \bool_if:NTF \l_figput_done_bool
  {
    \str_put_right:Nx \l_figput_filespec_str { ~ true }
  }
  {
    \str_put_right:Nx \l_figput_filespec_str { ~ false }
  }
  
  % And indicate that this function must appear in an external file.
  % It is not (false) to be loaded from a seperate file.
  \str_put_right:Nx \l_figput_filespec_str { ~ false }

  % String is ready. Write out the line.
  \iow_now:Nx \g_figurefile_iow { \l_figput_filespec_str }
  
  % That's it for the figures.aux file. Next is the tikz.
  % Above is identical to the environment code, but the following differs
  % slightly since there is no body.
  \bool_set_true:N \l_figput_readtikz_bool
  
  \bool_if:NTF \l_figput_skip_bool
  {
    \bool_set_false:N \l_figput_readtikz_bool
  }
  {}

  % In the environment definition, this was part of the "post body"
  % set of commands.
  \bool_if:NTF \l_figput_nostatic_bool
  {
    % nostatic is set, so do nothing at all -- no figure in the static version.
  }
  {
    % nostatic is not set, so there is a figure.
    \str_set_eq:NN \l_figput_tikzfile_str \l_figput_figname_str
    \str_put_right:Nn \l_figput_tikzfile_str { .tikz }

    % Whether to read the tikz file or display a "not available" message
    % depends on whether the tikz file exists and whether skip was set.
    \file_if_exist:VTF \l_figput_tikzfile_str
    {
      % Yes, tikz exists, although we may not read it if skip is set.
    }
    {
      % Tikz file doesn't exist, so we can't read it in any case.
      \bool_set_false:N \l_figput_readtikz_bool
    }
    
    \bool_if:NTF \l_figput_readtikz_bool
    {
      % Load the tikz file here.
      \file_input:V \l_figput_tikzfile_str
    } 
    {
      % Not loading the tikz file. Display a big empty box.
      \begin{tikzpicture}
        \useasboundingbox (0pt,0pt) rectangle (\textwidth,\l_figput_height_dim);
        \draw[dashed] (0pt,0pt) rectangle ( \textwidth,\l_figput_height_dim);
        \node at (\textwidth / 2,\l_figput_height_dim / 2) {The\ drawing\ is\ not\ available\ to\ load.};
        \draw (5pt,5pt) rectangle ( \textwidth - 5,\l_figput_height_dim - 5);
      \end{tikzpicture}
    }

    % End of figure inclusion -- nostatic is not set.
  }
  
  % Note the figures location as of after the figure.
  \par\zsaveposy { \l_figput_figname_str }
  \zlabel{ \l_figput_figname_str -pageno}

  \group_end:  
}



% See xparse for the top-level definition, \NewDocumentEnvironment and
% its use of \SplitArgument. The gist is that there will be two
% arguments, one for the manditory items (m), given in {}, and one for
% the optional arguments (o), given in [].
%
% In theory, it is possible to take the body of the environment as an
% argument, but that won't work if you want to receive the body
% verbatim. Somehow, using xsim allows the body to be taken verbatim.
%
% Note also the ! that appears before the o. This is explained in the
% xparse documentation. Without the !, if the user does not provide
% any optional arguments, then any leading space in the body will be
% eaten and disappear.

\NewDocumentEnvironment{figput} { > { \SplitArgument { 1 } { , } } m!o }
{
  % I don't understand latex's concept of local variable, but
  % this makes what follows local in some sense.
  % BUG: Does this even matter? It looks like all my variables are in
  % the figput module.
  \group_begin:

  % The manditory arguments come in in #1, but they have been
  % broken up so that #1 "reads as" {first arg}{second arg}. By
  % passing this to another function, it automatically breaks this
  % into two distinct arguments. This seems a silly way, but it works.
  % This sets \l_figput_figname_str and \l_figput_height_dim.
  \figput_manargs #1

  % Now the optional arguments. Start off with some default values.
  % It looks like the htAbove and htBelow values are blank by default,
  % which is what I want.
  \bool_set_false:N \l_figput_hasintht_bool
  \bool_set_false:N \l_figput_nostatic_bool
  \bool_set_false:N \l_figput_done_bool
  \bool_set_false:N \l_figput_skip_bool
  
  % Only try to set these values if there *were* some optional arguments,
  % which come in as the #2 argument to figput.
  % I set the clist here and "pass" it as global variable since I can't
  % figure out how to pass the #2 argument directly. This works, but it
  % is ugly. So this does nothing if the #2 argument is empty, leaving
  % the boolean values as set above. Otherwise, it reads in the
  % height adjustments and sets the boolean values above.
  \tl_if_blank:nTF { #2 } {} {
    \clist_set:Nn \l_figput_optclist_clist { #2 }
    \figput_optargs { } 
  }

  % We have everything needed to know what to do. Prepare to write the
  % data to the figures file. It seems that the easiest way to do this
  % is to build up the string, then write it out. Getting spaces right
  % is hard when I tried to write each bit directly to the file.
  % Unravelling what's actually written and its meaning is hard to do
  % here. It's easier to understand the js code that reads this output.
  
  % Need the position on the page. This saves it (for the next run).
  % Just as for the page number (see below), it turns out that this
  % shouldn't be done here; if it is, the figures at the bottom of the page are
  % improperly positioned. This is done *after* the environment closes.
  %\zsaveposy { \l_figput_figname_str }
  
  % And this gets the value from the previous run.
  % This value is relative to the bottom of the page (normal axes)
  % See below. The possibility of a figure at the bottom of the page
  % requires a bit of trickery.
  %\dim_set:Nn \l_figput_pagepos_dim { \zposy { \l_figput_figname_str } sp}

  % Getting the page number correct is fiddly. We need to place a
  % label at this location, and this label is updated at shipout.
  % Because I am using the figure name as the label for the vertical
  % position, I add a suffix ("-pageno") to distinguish this label.
  % Originally, I had this line here, but that doesn't work because
  % the page number is that at which the environment "opens" rather
  % than when it "closes." If the figure is at the bottom of the page,
  % then it seems that you get the page prior to the one you want.
  %\zlabel{ \l_figput_figname_str -pageno}

  % I would have thought that saying
  % \zref[abspage]{ \l_figput_figname_str }
  % would work, but it's not properly expanded. It *does* work to say
  % the above and have it appear as a number on the page. I don't fully
  % understand what the bit below does, but it works.
  \str_set:Nx \l_figput_filespec_str { 
    \use:c { zref@extractdefault } { \l_figput_figname_str -pageno} { abspage } { 0 }
  }

  % The inner and outer margins.
  \str_put_right:Nx \l_figput_filespec_str { ~ \dim_to_decimal_in_bp:n \l_figput_innermargin_dim }
  \str_put_right:Nx \l_figput_filespec_str { ~ \dim_to_decimal_in_bp:n \l_figput_outermargin_dim }

  % And the text width.
  \str_put_right:Nx \l_figput_filespec_str { ~ \dim_to_decimal_in_bp:n \textwidth }

  % Use tilde (~) to get a space in the string, and convert to bp (big
  % points). Fortunately, this eliminates the "bp" suffix too.
  % There is a problem here, related to whether the figure is at the
  % bottom of the page and gets bumped to the next page.
  % I want to know the location of the top of the figure relative to
  % the page on which it appears. The way to get the position on the
  % page is with \zsaveposy. The problem is that if I want this
  % location *before* the figure, then you get the bottom of the text
  % immediately before the figure; if you ask for this location
  % *after* the figure (or from within the figure), then you get the
  % location of the bottom edge of the figure.
  %
  % The solution is to get the location of the bottom edge of the
  % figure, then subtract the known figure height -- actually we *add*
  % because we are in "normal" coordinates. We want to move up on the
  % page.
  %
  % So, get the value from the previous run, in sp (small or scaled points).
  \tl_set:Nn \l_figput_tempa_tl { \zposy { \l_figput_figname_str } }
  % Convert the figure height to sp.
  \tl_set:Nn \l_figput_tempb_tl { \dim_to_decimal_in_sp:n{ \l_figput_height_dim } }
  % Add 
  \tl_set:Nn \l_figput_tempc_tl { \int_eval:n { \l_figput_tempa_tl + \l_figput_tempb_tl } }
  % Convert to a dim
  \dim_set:Nn \l_figput_pagepos_dim { \l_figput_tempc_tl sp }
  % And (whew!) write it out, in bp.
  \str_put_right:Nx \l_figput_filespec_str { ~ \dim_to_decimal_in_bp:n \l_figput_pagepos_dim }

  % The latex height of the figure (in bp)
  \str_put_right:Nx \l_figput_filespec_str { ~ \dim_to_decimal_in_bp:n \l_figput_height_dim }

  % The interactive height paddings, which are zero by default.
  \bool_if:NTF \l_figput_hasintht_bool
  { 
    \str_put_right:Nx \l_figput_filespec_str { ~ \dim_to_decimal_in_bp:n \l_figput_htAbove_dim }
    \str_put_right:Nx \l_figput_filespec_str { ~ \dim_to_decimal_in_bp:n \l_figput_htBelow_dim }
  }
  {
    \str_put_right:Nx \l_figput_filespec_str { ~0~0 }
  }

  % The figure name -- used for the JS function to call.
  \str_put_right:Nx \l_figput_filespec_str { ~ \l_figput_figname_str }

  % And a boolean
  \bool_if:NTF \l_figput_done_bool
  {
    \str_put_right:Nx \l_figput_filespec_str { ~ true }
  }
  {
    \str_put_right:Nx \l_figput_filespec_str { ~ false }
  }

  % We're making a seperate file for this particular function.
  % Tell the browser (true) to load it.
  \str_put_right:Nx \l_figput_filespec_str { ~ true }
  
  % String is ready. Write out the line.
  \iow_now:Nx \g_figurefile_iow { \l_figput_filespec_str }
  
  % That's it for the fig.aux file. Next is the heavy lifting,
  % making the environment shift blocks of text around. Exactly what
  % to do depends on the optional flags.
  %
  % Ultimately, there are only two things we might do (plus nothing):
  % write the body of the environment to a file, and/or read a tikz
  % file in to replace the current body (which may be blank). The
  % easiest way to manage these options is to set a couple of booleans
  % to indicate whether we should do these two things.
  \bool_set_true:N \l_figput_writebody_bool
  \bool_set_true:N \l_figput_readtikz_bool

  % Note that there is no need to check nostatic here.
  
  \bool_if:NTF \l_figput_skip_bool
  {
    % skip == true, so don't write body or read tikz.
    \bool_set_false:N \l_figput_writebody_bool
    \bool_set_false:N \l_figput_readtikz_bool
  }
  {}

  \bool_if:NTF \l_figput_writebody_bool
  {
    % Yes, we are to write the body to an external file.
    
    % The output file name needs an extra ".fjs" on the end.
    % I don't like using this suffix, but it's seems the least bad.
    \str_set_eq:NN \l_figput_jinfile_str \l_figput_figname_str
    \str_put_right:Nn \l_figput_jinfile_str { .fjs }
  
    % Whether to add an additional EOL to the output file depends on
    % whether there were any optional arguments. That's what the boolean
    % argument does here. It tells the writer whether to start off with
    % an "extra" EOL.
    \IfValueTF {#2}
      { \xsim_file_write_start:nn { \c_true_bool } }
      { \xsim_file_write_start:nn { \c_false_bool } }
      { \l_figput_jinfile_str }
  }
  {
    % Do not write the body to an external file, but it still needs
    % to be "eaten" and thrown away; otherwise, it passes through
    % and latex tries to make the JS code into part of the document.
    % This uses the verbatim package.
    \comment
  }
}{
  % Post environment commands. Remember that the "execution" of an
  % environment is divided into two parts.
  \bool_if:NTF \l_figput_writebody_bool
  {
    % Stop writing the body since it's done.
    \xsim_file_write_stop:
  }
  {
    \endcomment
  }

  \bool_if:NTF \l_figput_nostatic_bool
  {
    % nostatic is set, so do nothing at all -- no figure in the static version.
  }
  {
    % nostatic is not set, so there is a figure.

    % And replace the current body with an external tikz file.
    \str_set_eq:NN \l_figput_tikzfile_str \l_figput_figname_str
    \str_put_right:Nn \l_figput_tikzfile_str { .tikz }

    % Whether to read the tikz file or display a "not available" message
    % depends on whether the tikz file exists and whether skip was set.
    \file_if_exist:VTF \l_figput_tikzfile_str
    {
      % Yes, tikz exists, although we may not read it if skip is set.
    }
    {
      % Tikz file doesn't exist, so we can't read it in any case.
      \bool_set_false:N \l_figput_readtikz_bool
    }
    
    \bool_if:NTF \l_figput_readtikz_bool
    {
      % Load the tikz file here.
      \file_input:V \l_figput_tikzfile_str
    } 
    {
      % Not loading the tikz file. Display a big empty box.
      % The problem here is that I need to specify the size of the
      % drawing, and tizk doesn't make that easy. It prefers to look at
      % the drawing and determine the size based on what was actually
      % drawn. It seems as though it is possible to *scale* a drawing to
      % take a particular size, although simply specifying the size is
      % hard. What does seem to work is one of the two following approaches.
      % (1) If you want to "trick" latex into allocating a given area
      % for the drawing, even if the drawing ends up going "outside the
      % lines," then you can say (within the tikzpicture environment)
      % \useasboundingbox (0pt,0pt) rectangle (100pt,200pt);
      % or whatever the dimensions are. Use this on the first line.
      % (2) This idea doesn't so much trick latex into using a particular
      % vbox/hbox for the drawing as limit the drawing to the rectangle
      % you want by clipping it. Here, as the first line of the drawing
      % you say
      % \clip (0,0) rectangle (100pt,200pt);
      % and the drawing will be limited to that region. I suspect (?)
      % that if the actual drawing is smaller than the clip region, the
      % latex will reduce the size of the figure accordiningly. So (1)
      % is probably the better solution.
      
      \begin{tikzpicture}
        % The opposite corner is given by (width,height), and the width
        % doesn't really matter since I assume that the figure spans the page.
        % BUG: Maybe I should change this to bp?
        % BUG: The right thing to do here is (?) to make the box match
        % the settings for \l_figput_innermargin_dim and outermargin, but
        % I think this requires that I know the page width.
        \useasboundingbox (0pt,0pt) rectangle (\textwidth,\l_figput_height_dim);
        
        % I don't understand exactly how the figure is placed on the
        % page, but this seems to work.
        \draw[dashed] (0pt,0pt) rectangle ( \textwidth,\l_figput_height_dim);
        
        % It looks like the text is drawn so that the bounding box is
        % centered at the given coordinates. Note how tikz lets you
        % divide with a simple "/2". Nice.
        %
        % Note also that, because this is being done with \ExplSyntaxOn,
        % the spaces in the text need to be \-spaces. Also, if I say
        % \node[draw], then a box is drawn around the text.
        \node at (\textwidth / 2,\l_figput_height_dim / 2) {The\ drawing\ is\ not\ available\ to\ load.};
      
        % For visual appeal, an extra enclosing rectangle.
        % BUG: I might want to check how tall the figure is and adjust
        % this box accordingly, or not show it at all if the box is very small.
        \draw (5pt,5pt) rectangle ( \textwidth - 5,\l_figput_height_dim - 5);

      \end{tikzpicture}
    }

    % End of figure inclusion -- nostatic is not set.
  }
  
  % See above. I originally had these lines near the start of the
  % process, but I had to put it here, as the very last thing, for the
  % values to be correct. Otherwise figures at the very end of a
  % page are assigned the page number prior to what it should be, and
  % they end up on the bottom of that page.
  %
  % Wow, I really don't understand this, but somehow, adding the \par
  % here serves to "quiet" another \par that would be inserted and
  % that I do not want. Without this, it seems like there is an extra
  % \par following the .tikz file when one is loaded. This solution
  % was provided as the answer to my question 643125 on tex.stackexchagne.
  % Apparently, it has something to do with \zsaveposy being a
  % "whatsit" command. I haven't unravelled this fully, but it somehow
  % explains what is going on.
  \par\zsaveposy { \l_figput_figname_str }
  \zlabel{ \l_figput_figname_str -pageno}

  % Note that group_end occurs here. It can't appear above, in the
  % pre-environment stuff because we still need access to some of the
  % variables here, post-environment.
  \group_end:  
}

% I wanted to define this with \cs_new, but lost patience with it.
% This works, but it's probably not "the right way."
\NewDocumentCommand{\figput_manargs}{ m m }
{
  % You need braces around #1 or you only get the first character.
  \str_set:Nn \l_figput_figname_str { #1 }
  \dim_set:Nn \l_figput_height_dim { #2 }
}

% This is to handle the optional arguments. It merely sets the
% relevant variables. 
\NewDocumentCommand{\figput_optargs} { }
{
  % Certain things are hard to do using a clist, so it's held as a seq
  % too. Again, no doubt there is a  better way, but this works.
  \seq_set_from_clist:NN \l_figput_optseq_seq { \l_figput_optclist_clist }
  
  % The first of the optional arguments might be a length. If it is, then
  % we have two lengths (dims), for the additional space above/below when
  % the figure is viewed interactively.
  % 
  % Note that one could make the call directly from the entire set of
  % optional arguments with
  %\figput_if_length:eTF { \clist_item:nn { #2 } { 1 } } {TRUE}{FALSE} \\
  % However (!) this only works if the function if_length is defined
  % with \prg_new_conditional:Nnn rather than
  % \prg_new_protected_conditional. I have no idea what the difference
  % is.
  %
  % Pull out the first of the optional arguments to make it easier to
  % work with. Note that by using a copy of the data in a seq variable,
  % I can pop things off without worrying about side-effects.
  \seq_pop_left:NN \l_figput_optseq_seq \l_figput_firstvalue_tl

  % If the first value parses out to be a dimension, then note that
  % value as the height above and get the height below too.
  \figput_if_length:VTF \l_figput_firstvalue_tl 
  {
    \dim_set:Nn \l_figput_htAbove_dim \l_figput_firstvalue_tl
    \bool_set_true:N \l_figput_hasintht_bool
    
    % Get the second value (reusing the "firstvalue" variable).
    \seq_pop_left:NN \l_figput_optseq_seq \l_figput_firstvalue_tl
    \dim_set:Nn \l_figput_htBelow_dim \l_figput_firstvalue_tl
  }
  {
    % If it's not a dimension, then ignore it for now.
  }

  % Check whether each possible boolean flag has been set.
  % BUG: I should check whether the user passed in something
  % spurious, and he might accidentally indicate the interactive
  % heights out of order. As things stand now, the user could pass in
  % "gibberish" as an option and it would be ignored. No harm done,
  % but it could confuse the user.
  \clist_if_in:NnTF \l_figput_optclist_clist { nostatic } 
  {
    \bool_set_true:N \l_figput_nostatic_bool
  }{}
  \clist_if_in:NnTF \l_figput_optclist_clist { done } 
  {
    \bool_set_true:N \l_figput_done_bool
  }{}
  \clist_if_in:NnTF \l_figput_optclist_clist { skip } 
  {
    % Note the test for whether \NeverSkip was called.
    \bool_if:NTF \l_figput_skipoff_bool
    {}{
      \bool_set_true:N \l_figput_skip_bool
    }
  }{}
}

% This was a "top-level" command, defined with
%\NewDocumentCommand{\iflengthTF}{mmm}
% but that's really not the way to do this.
%
% Instead, we define a function "in the module" (figput), plus a
% "variant." See p. 32 of the interface3 document.  For the initial
% function (not the variant), see p. 62. I can't say that I really
% understand what's going on here, other than a bit of copy and paste
% and the help from stack overflow does it.
\prg_new_protected_conditional:Nnn \figput_if_length:n { T, F, TF }
 {
  \regex_match:nnTF
  % Note that I only allow positive values.
   { \A [+]? ((\d+(\.\d*)?)|(\.\d+)) \s* (pt|pc|in|bp|cm|mm|dd|cc|sp|ex|em) \Z} 
   { #1 } % test string
   { \prg_return_true: }
   { \prg_return_false: }
 }
 
\prg_generate_conditional_variant:Nnn \figput_if_length:n { V } { T, F, TF }


\ExplSyntaxOff