%D \module
%D   [       file=math-stc,
%D        version=2012.12.29,
%D          title=\CONTEXT\ Math Macros,
%D       subtitle=Stackers,
%D        comment=This replaces math-arr and friends,
%D         author=Hans Hagen,
%D           date=\currentdate,
%D      copyright={PRAGMA ADE \& \CONTEXT\ Development Team}]
%C
%C This module is part of the \CONTEXT\ macro||package and is
%C therefore copyrighted by \PRAGMA. See mreadme.pdf for
%C details.

\writestatus{loading}{ConTeXt Math Macros / Stackers}

% todo mathmeaning like fractions

\unprotect

% is subtype for tagged needed: dosettagproperty only used here .. maybe for tagged pdf?

%D WARNING: If the code here changes, the export needs to be checked! Stackers are
%D rather special because the order in mathml matters, so we flush in [base under
%D over] order. We also do some analysis at the \TEX\ end (passing the right
%D variant). It's easy in the export to deal with it but in the pdf stream less
%D trivial as we don't actually analyze there.
%D
%D At some point the \MKII\ arrow mechanism has been converted to \MKIV, but we kept
%D most of the logic. We now have a more generic variant dealing with extensibles.
%D There are a few demands than we need to meet:
%D
%D \startitemize
%D \startitem
%D    The width of the extensible need to adapt itself automatically.
%D \stopitem
%D \startitem
%D   We need to be able to control horizontal and vertical offsets.
%D \stopitem
%D \startitem
%D   We best have a math as well as a text variant (which is handy for chemistry).
%D \stopitem
%D \startitem
%D    For historic reasons we need to deal with optional arguments in a special
%D    (reverse) way.
%D \stopitem
%D \startitem
%D    We need alternatives for extensibles on top, in the middle and at the bottom.
%D \stopitem
%D \stopitemize
%D
%D After I had experimented a bit with virtual characters for two headed arrows I
%D discussed the issue with the Gyre folks and we came to the conclusion that it
%D made sense to have real extensibles instead of constructing them out of snippets.
%D After all, \OPENTYPE\ math provides for it. So, in December 2013 beta versions of
%D Latin Modern and Gyre fonts came available that had these! Because we still want
%D to support the traditional Latin Modern Virtual math font those were extended
%D with a couple of virtual extensibles as well.
%D
%D {\em For the moment we still have some mess here: we can deal with known
%D dimensions, but fillers (like \type {\rightarrowfil} don't work with \OPENTYPE\
%D extensibles yet because there is no way to let them stretch like leaders. At some
%D point \LUATEX\ might provide a auto||fit||to||encapsulated||box and if not I will
%D cook up a \LUA\ based variant.}
%D
%D We could mess with something like \type {$mid\limits^{top}_{bottom}$} but we like
%D a bit more control. At some point we need to add some hacks to get exports
%D working well.
%D
%D In the end we have a more flexible mechanism which also handles text variants.

%D When wrapping up some math developments I decided to add mp support here as well.
%D A nice evening job with Joe Bonamassa performing live on the big screen (real
%D nice bluray's). See meta-imp-mat.mkiv for examples.

% possible improvements:
%
% - we could skip the left/right offsets when offset=normal, this saves some access time
%   at the lua end and some checking: use \mathhorizontalcode or \mathextensiblecode
%   but in practice arrows etc are not used that often

% At some point we can consider to use the more natural \LUAMETATEX\ features but the
% problem is that we lack proper support in fonts and we also have less control.

%installcorenamespace {mathextensiblefallbacks}

% In math mode we no longer fallback, simply because only a few fonts implement
% extensible using minus and equal signs. So either we provide a tweaked one or we
% simply ignore the lack. Better choose a font that matches expectations that some
% fragile imperfect hackery.

%D The extensible gets sort of inlined so we loose tagging applied to the box
%D which actually is an nested mlist_to_list call then. Kind of messy in the
%D tagging code.

\def\math_stackers_fallback
  {\mathstylehbox to \scratchwidth{% no \bgroup ... \egroup
     \usemathstackerscolorparameter\c!color
     \hss
     \hskip{\mathstackersparameter\c!topoffset}% for manual italic correction
   % \ifcsname\??mathextensiblefallbacks\the\scratchunicode\endcsname
   %   \lastnamedcs
   % \else
       \Umathchar\zerocount\zerocount\scratchunicode
   % \fi
     \hss}}

\def\math_stackers_regular
  {\mathstylehbox{%
     \hskip\d_math_stackers_offset_l
     \ifcstok{\mathstackersparameter\c!plugin}\v!mp
       \edef\p_mp{\mathstackersparameter\c!mp}%
       \ifempty\p_mp\else
         \clf_set_extensible_data
             mp      {\p_mp}
             unicode \scratchunicode
          \relax
       \fi
     \else
       \lettonothing\p_mp
     \fi
     \Uhextensible
       \usedsymbolcolorparameterattributes{\mathstackersparameter\c!symbolcolor}%
       \ifempty\p_mp\else
         \s!usecallback\relaxedspace
       \fi
       middle
       \ifcstok{\mathstackersparameter\c!stretch}\v!yes
         stretch
       \fi
       \ifcstok{\mathstackersparameter\c!shrink}\v!yes
         shrink
       \fi
       width {\scratchwidth-\d_math_stackers_offset_l-\d_math_stackers_offset_r}%
       \zerocount\scratchunicode
     \relax
     \hskip\d_math_stackers_offset_r
   }}

% these delimiters are a unuseable as they don't center for small arguments:
%
% $\Umathaccent   0 0 "2190{x}$ \par $\Umathaccent   0 0 "27F8{x}$\par
% $\Udelimiterunder 0 "2190{x}$ \par $\Udelimiterunder 0 "27F8{x}$\par

% \defcsname\??mathextensiblefallbacks\endcsname
%   {\hpack{\vrule\s!width\scratchwidth\s!height.1mx\s!depth\zeropoint}}

%D We don't really need this because we can assume that fonts have the right
%D extensibles. If needed I will make a general virtual extender for \OPENTYPE\
%D fonts.
%D
%D Because we have quite some control over positioning, we have somewhat extensive
%D tracing built in.

\let\math_stackers_top   \relax
\let\math_stackers_middle\relax
\let\math_stackers_bottom\relax
\let\math_stackers_skip  \hskip

\installtextracker
  {math.stackers.texts}
  {\let\math_stackers_top   \filledhboxb
   \let\math_stackers_middle\filledhboxr
   \let\math_stackers_bottom\filledhboxg
   \let\math_stackers_skip  \math_stackers_skip_indeed}
  {\let\math_stackers_top   \relax
   \let\math_stackers_middle\relax
   \let\math_stackers_bottom\relax
   \let\math_stackers_skip  \hskip}

\def\math_stackers_skip_indeed#amount%
  {\filledhboxk{\strut\hskip#amount}}

\let         \math_stackers_start_tagged_mid\gobbleoneargument
\let         \math_stackers_start_tagged_top\gobbleoneargument
\let         \math_stackers_start_tagged_bot\gobbleoneargument
\lettonothing\math_stackers_stop_tagged

\def\math_stackers_unicode{\ifcase\scratchunicode\else\the\scratchunicode\fi}

\mutable\lettonothing\math_stackers_unicode_two
\mutable\lettonothing\math_stackers_unicode_one

\def\math_stackers_reset_variables
  {\scratchunicode\zerocount
   \edef\p_offset     {\mathstackersparameter\c!offset}%
   \edef\p_location   {\mathstackersparameter\c!location}%
   \edef\p_strut      {\mathstackersparameter\c!strut}%
   \edef\p_alternative{\mathstackersparameter\c!alternative}%
   \scratchleftoffset \zeropoint
   \scratchrightoffset\zeropoint}

\appendtoks
    \def\math_stackers_start_tagged_mid#1{\dostarttagged\t!mstackermid{#1}\hbox\bgroup}%
    \def\math_stackers_start_tagged_top#1{\dostarttagged\t!mstackertop{#1}\hbox\bgroup}%
    \def\math_stackers_start_tagged_bot#1{\dostarttagged\t!mstackerbot{#1}\hbox\bgroup}%
    \def\math_stackers_stop_tagged       {\egroup\dostoptagged}%
\to \everysetuptagging

%D We define a full featured command handler.

\installcorenamespace {mathstackers}

\installcommandhandler \??mathstackers {mathstackers} \??mathstackers

% ma (axis) mx (xheight) mq (quad)

\setupmathstackers
  [%c!alternative=\v!text,    % text | mathematics
   \c!left=,
   \c!right=,
   \c!mathclass=\s!relation,
   \c!alternative=\v!normal,
   \c!voffset=.25mx, % maybe less
   \c!hoffset=\zeropoint,
   \c!topoffset=\zeropoint, % for manual italic correction
   \c!distance=\mathstackersparameter\c!voffset, % distance between symbol and base (can be different from voffset)
   \c!minheight=1mx, % it has to be adaptive
   \c!mindepth=\zeropoint,
   \c!minwidth=.25mq, % \iota
   \c!order=\v!normal,
   \c!strut=,
   \c!color=,
   \c!symbolcolor=\mathstackersparameter\c!color,
   \c!topcommand=,
   \c!middlecommand=,
   \c!bottomcommand=,
   \c!offset=\v!normal,       % normal | min | max
   \c!location=\v!top]        % none | normal | small | medium | big

%D We assume that the middle characters (that can be an extensible) to sit on
%D top of the baseline by default.

\installcorenamespace {mathstackerslocation}
\installcorenamespace {mathstackersalternative}

\letcsname\??mathstackerslocation\v!top    \endcsname\plusone   % on top of baseline
\letcsname\??mathstackerslocation\v!high   \endcsname\plustwo   % 25 % down
\letcsname\??mathstackerslocation\v!middle \endcsname\plusthree % centered
\letcsname\??mathstackerslocation\v!low    \endcsname\plusfour  % 75 % down
\letcsname\??mathstackerslocation\v!bottom \endcsname\plusfive  % below baseline
\letcsname\??mathstackerslocation          \endcsname\zerocount

%D First we implement the helper that deals with an extensible in the middle and
%D top and|/|or bottom texts:

\lettonothing\m_math_stackers_text_top
\lettonothing\m_math_stackers_text_bottom
\lettonothing\m_math_stackers_text_middle

\def\math_stackers_flushtext#command#style#color#text%
  {\ifdim\scratchleftoffset>\zeropoint
     \math_stackers_skip\scratchleftoffset
   \fi
   \ifx\p_strut\v!no \else
     \strut
   \fi
   \usemathstackersstyleandcolor#style#color%
   \mathstackersparameter#command#text%
   \ifdim\scratchrightoffset>\zeropoint
     \math_stackers_skip\scratchrightoffset
   \fi}

\def\math_stackers_toptext   {\math_stackers_flushtext\c!topcommand   \c!topstyle   \c!topcolor   \m_math_stackers_text_top   }
\def\math_stackers_bottomtext{\math_stackers_flushtext\c!bottomcommand\c!bottomstyle\c!bottomcolor\m_math_stackers_text_bottom}
\def\math_stackers_middletext{\math_stackers_flushtext\c!middlecommand\c!middlestyle\c!middlecolor\m_math_stackers_text_middle}

\def\math_stackers_content
  {\ifcase\scratchcounter
     \math_stackers_fallback
   \or % left
     \math_stackers_regular
   \or % right
     \math_stackers_regular
   \or % horizontal
     \math_stackers_regular
   \else
     \math_stackers_fallback
   \fi}

% no checking, we assume sane use

\letcsname\??mathstackersalternative\v!normal \endcsname\math_stackers_content
\letcsname\??mathstackersalternative\v!default\endcsname\math_stackers_content

\setupmathstackers
  [\c!mp=math:stacker:\the\scratchunicode,
   \c!mpheight=\mathcharht\scratchunicode,
   \c!mpdepth=\mathchardp\scratchunicode,
   \c!mpoffset=.25mx]

% for now we do this:

\setupmathstackers
  [\c!mp=\ifcstok{\mathstackersparameter\c!plugin}\v!mp simplefun::math:default\else math:stacker:\the\scratchunicode\fi]

% so far

\defcsname\??mathstackersalternative\v!mp\endcsname
  {\expanded{\math_stackers_mp_box
     {\todimension{\mathstackersparameter\c!mpheight}}%
     {\todimension{\mathstackersparameter\c!mpdepth}}%
     {\todimension{\mathstackersparameter\c!mpoffset}}%
     {\todimension{\triggeredmathstyleparameter\Umathfractionrule}}%
     {\todimension{\triggeredmathstyleparameter\Umathaxis}}%
     {\todimension1mx}% maybe also triggered?
     {\todimension1mq}% maybe also triggered?
   }}

\protected\def\math_stackers_mp_box#1#2#3#4#5#6#7%
  {\hpack\bgroup %  todo: add code key + tag
   \d_overlay_width    \scratchwidth
   \d_overlay_height   {#1}%
   \d_overlay_depth    {#2}%
   \d_overlay_offset   {#3}%
   \d_overlay_linewidth{#4}%
   \edef\overlaylinecolor{\mathstackersparameter\c!color}%
   \edef\p_mp{\mathstackersparameter\c!mp}%
   \uniqueMPgraphic{\p_mp}{axis=#5,ex=#6,em=#7}%
   \egroup}

\def\math_stackers_check_unicode#codepoint%
  {\scratchunicode#codepoint\relax
   \scratchhoffset{\mathstackersparameter\c!hoffset}%
   \scratchvoffset{\mathstackersparameter\c!voffset}%
   \scratchcounter\mathhorizontalcode\fam\scratchunicode\relax % also sets \leftscratchoffset and \rightscratchoffset
   \ifx\p_offset\v!max
     % heads/tails + hoffset
   \orelse\ifx\p_offset\v!min
     % heads/tails - hoffset
     \advanceby\scratchleftoffset -\scratchhoffset
     \advanceby\scratchrightoffset-\scratchhoffset
   \else % \v!normal
     % hoffset
     \scratchleftoffset\zeropoint
     \scratchrightoffset\zeropoint
   \fi
   \ifdim\scratchleftoffset<\zeropoint
     \scratchleftoffset\zeropoint
   \fi
   \ifdim\scratchrightoffset<\zeropoint
     \scratchrightoffset\zeropoint
   \fi}

\def\math_stackers_normalize_three
  {\scratchheight\ht\scratchboxthree
   \scratchdepth \dp\scratchboxthree
   \scratchtopoffset   \scratchheight
   \scratchbottomoffset\scratchdepth
   \scratchdimen{\mathstackersparameter\c!minheight}%
   \ifdim\scratchheight<\scratchdimen
     \scratchheight\scratchdimen
     \ht\scratchboxthree\scratchheight
   \fi
   \scratchdimen{\mathstackersparameter\c!mindepth}%
   \ifdim\scratchdepth<\scratchdimen
     \scratchdepth\scratchdimen
     \dp\scratchboxthree\scratchdepth
   \fi
   \advanceby\scratchtopoffset   -\scratchheight
   \advanceby\scratchbottomoffset-\scratchdepth
   \ifdim\scratchtopoffset<\zeropoint
     \scratchtopoffset\zeropoint
   \fi
   \ifdim\scratchbottomoffset<\zeropoint
     \scratchbottomoffset\zeropoint
   \fi}

\def\math_stackers_get_max_width
  {\ifdim\wd\scratchboxone>\scratchwidth
     \scratchwidth\wd\scratchboxone
   \fi
   \ifdim\wd\scratchboxtwo>\scratchwidth
     \scratchwidth\wd\scratchboxtwo
   \fi
   \ifdim\wd\scratchboxthree>\scratchwidth
     \scratchwidth\wd\scratchboxthree
   \fi}

\def\math_stackers_set_max_width
  {\ifdim\wd\scratchboxone<\scratchwidth
     \setbox\scratchboxone\hpack to \scratchwidth{\hss\unhbox\scratchboxone\hss}% unhboxing makes leaders work
   \fi
   \ifdim\wd\scratchboxtwo<\scratchwidth
     \setbox\scratchboxtwo\hpack to \scratchwidth{\hss\unhbox\scratchboxtwo\hss}%
   \fi
   \ifdim\wd\scratchboxthree<\scratchwidth
     \setbox\scratchboxthree\hpack to \scratchwidth{\hss\unhbox\scratchboxthree\hss}%
   \fi}

\protected\def\math_stackers_triplet#method#category#codepoint#toptext#bottomtext%
 %{\math_stackers_start_group{#category}%
  {\begingroup
   \cdef\currentmathstackers{#category}%
   \mathstackersparameter\c!left\relax
   \dostarttagged\t!mstacker\currentmathstackers
   \ifmmode\math_atom_by_parameter\mathstackersparameter\else\dontleavehmode\fi
     {\math_stackers_reset_variables
      \edef\m_math_stackers_text_top   {#toptext}%
      \edef\m_math_stackers_text_bottom{#bottomtext}%
      \ifparameter#bottomtext\or
        \ifcstok{\mathstackersparameter\c!order}\v!reverse
          \swapmacros\m_math_stackers_text_top\m_math_stackers_text_bottom
        \fi
      \fi
      \ifcase#method\relax
        \math_stackers_check_unicode{#codepoint}%
      \else
       %\scratchunicode\zerocount
        \edef\m_math_stackers_text_middle{#codepoint}%
      \fi
      \ifempty\m_math_stackers_text_top
        \setbox\scratchboxone\emptyhbox
      \else
        \setmathsmalltextbox\scratchboxone\hbox{\math_stackers_toptext}%
      \fi
      \ifempty\m_math_stackers_text_bottom
        \setbox\scratchboxtwo\emptyhbox
      \else
        \setmathsmalltextbox\scratchboxtwo\hbox{\math_stackers_bottomtext}%
      \fi
      %
      \ifcase#method\relax
        % e.g. extensible
       %\scratchwidth\wd
       %  \ifdim\wd\scratchboxone>\wd\scratchboxtwo
       %    \scratchboxone
       %  \else
       %    \scratchboxtwo
       %  \fi
       %\relax
        \scratchwidth\mathcharwd\scratchunicode
        \ifdim\wd\scratchboxone>\scratchwidth
          \scratchwidth\wd\scratchboxone
        \fi % no \elseif here
        \ifdim\wd\scratchboxtwo>\scratchwidth
          \scratchwidth\wd\scratchboxtwo
        \fi
      \else
        \ifempty\m_math_stackers_text_middle
          \setbox\scratchboxthree\emptyhbox
        \else
          \dostarttaggednodetail\t!mstackermid
            \setmathtextbox\scratchboxthree\hbox{\math_stackers_middletext}%
          \dostoptagged
        \fi
        \math_stackers_get_max_width
      \fi
      %
      \scratchdimen{\mathstackersparameter\c!minwidth}%
      \ifdim\scratchwidth<\scratchdimen
         \scratchwidth\scratchdimen
      \fi
      \advanceby\scratchwidth2\scratchhoffset
      %
      \ifcase#method\relax
        \dostarttaggednodetail\t!mstackermid
        \setbox\scratchboxthree\csname\??mathstackersalternative\p_alternative\endcsname
        \dostoptagged
      \fi
%       \ifdim\wd\scratchboxthree>\scratchwidth
%         \scratchwidth\wd\scratchboxthree
%       \fi
      %
      \math_stackers_set_max_width
      %
      \ifcsname\??mathstackerslocation\p_location\endcsname
        \ifcase\csname\??mathstackerslocation\p_location\endcsname\relax
          \scratchdistance\zeropoint
        \or % top
          \scratchdistance\zeropoint
        \or % high
          \scratchdistance.25\htdp\scratchboxthree
        \or % centered
          \scratchdistance.5\htdp\scratchboxthree
        \or % low
          \scratchdistance.75\htdp\scratchboxthree
        \or % bottom
          \scratchdistance\htdp\scratchboxthree
        \else
          \scratchdistance\zeropoint
        \fi
      \else
        \scratchdistance\p_location\htdp\scratchboxthree
      \fi
      %
      \ifzeropt\scratchdistance\else
        \setbox\scratchboxthree\hpack{\lower\scratchdistance\box\scratchboxthree}%
      \fi
      \math_stackers_normalize_three
      % analysis: is this still ok here?
      \ifdim\htdp\scratchboxtwo>\zeropoint
        \ifdim\htdp\scratchboxone>\zeropoint
          \dosettagproperty\s!subtype\t!munderover
        \else
          \dosettagproperty\s!subtype\t!munder
        \fi
      \else
        \ifdim\htdp\scratchboxone>\zeropoint
          \dosettagproperty\s!subtype\t!mover
        \else
          % brrr
        \fi
      \fi
      % base
      \math_stackers_start_tagged_mid\math_stackers_unicode
        \math_stackers_middle\bgroup
          \box\scratchboxthree
        \egroup
      \math_stackers_stop_tagged
      % under
      \ifdim\htdp\scratchboxtwo>\zeropoint
        \math_stackers_start_tagged_bot\empty
          \scratchoffset\scratchvoffset
          \kern-\scratchwidth
          \math_stackers_bottom\bgroup
            \lower{%
              \ht\scratchboxtwo+\scratchdepth+\scratchoffset+\scratchbottomoffset
            }\box\scratchboxtwo
          \egroup
        \math_stackers_stop_tagged
      \fi
      % over
      \ifdim\htdp\scratchboxone>\zeropoint
        \math_stackers_start_tagged_top\empty
          \scratchoffset\scratchvoffset
          \kern-\scratchwidth
          \math_stackers_top\bgroup
            \raise{%
              \dp\scratchboxone+\scratchheight+\scratchoffset+\scratchtopoffset
            }\box\scratchboxone
          \egroup
        \math_stackers_stop_tagged
      \fi
      %
      }%
  \dostoptagged
  \mathstackersparameter\c!right\relax
  \endgroup}
 %\math_stackers_stop_group}

\permanent\tolerant\protected\def\definemathextensible[#1]#*[#2]#*[#3]% category name unicode
  {\ifarguments\or\or
     \frozen\protected\edefcsname#1\endcsname{\math_stackers_auto_normal\noexpand\currentmathstackers{\number#2}}%
   \or
     \frozen\protected\edefcsname#2\endcsname{\math_stackers_auto_normal{#1}{\number#3}}%
   \fi}

% \tolerant\protected\def\math_stackers_auto_normal#1#2#*[#3]#:#*#=#*#=%
%   {\begingroup
%    \scratchcounter#2\relax
%    \cdef\currentmathstackers{\ifparameter#3\or#3\else#1\fi}%
%    \math_stackers_triplet\zerocount\currentmathstackers\scratchcounter{#4}{#5}%
%    \endgroup}

\tolerant\protected\def\math_stackers_auto_normal#1#2#*[#3]#:#*#=#*#=%
  {\begingroup
   \scratchcounter#2\relax
   \cdef\currentmathstackers{#1}%
   \ifhastok={#3}%
     \setupcurrentmathstackers[#3]%
   \orelse\ifparameter#3\or
     \cdef\currentmathstackers{#3}%
   \fi
   \math_stackers_triplet\zerocount\currentmathstackers\scratchcounter{#4}{#5}%
   \endgroup}

%D A few direct accessors (in the meantime we redefined \mathextensible so we renamed the
%D following):

\permanent\tolerant\protected\def\directmathextensible[#category]%
  {\begingroup
   \math_stackers_handle_extensible{\ifparameter#category\or#category\else\v!mathematics\fi}} % will be defined later on

\permanent\tolerant\protected\def\directtextextensible[#category]%
  {\begingroup
   \math_stackers_handle_extensible{\ifparameter#category\or#category\else\v!text\fi}} % will be defined later on

\aliased\let\mathstacker\directmathextensible
\aliased\let\textstacker\directtextextensible

\def\math_stackers_handle_extensible#category#codepoint#toptext#bottomtext%
  {\math_stackers_triplet\zerocount{#category}{#codepoint}{#toptext}{#bottomtext}%
   \endgroup}

% 1 0 name n 0 | 0 1 name n 0 | 1 1 name n n

\let\math_stackers_stop_group\endgroup

\protected\def\math_stackers_start_group#category%
  {\begingroup
   \cdef\currentmathstackers{#category}%
   \ifcstok{\mathstackersparameter\c!mathlimits}\v!yes
     \def\math_stackers_stop_group{\egroup\endgroup\ordlimits}%
   % \mathop\bgroup
     \mathaccent\bgroup
   \else
     \let\math_stackers_stop_group\endgroup
   \fi}

\newconstant \c_math_stackers_top
\newconstant \c_math_stackers_bottom
\newconstant \c_math_stackers_codepoint
\newconstant \c_math_stackers_extracode
\newdimension\d_math_stackers_offset_l
\newdimension\d_math_stackers_offset_r

\setupmathstackers[lt=\zeropoint,rt=\zeropoint,lb=\zeropoint,rb=\zeropoint]

\newbox\b_math_stackers_top_text
\newbox\b_math_stackers_bottom_text

\def\math_stackers_double_reset_texts
  {\global\setbox\b_math_stackers_top_text   \emptybox
   \global\setbox\b_math_stackers_bottom_text\emptybox}

\def\math_stackers_double_set_toptext
  {\edef\math_stackers_toptext{\mathstackersparameter\c!top}%
   \ifempty\math_stackers_toptext\else
     \global\setbox\b_math_stackers_top_text\hbox to \scratchwidth\bgroup
       \usemathstackersstyleandcolor\c!topstyle\c!topcolor
       \spac_align_simple{\mathstackersparameter\c!topalign}\math_stackers_toptext
      \hskip\d_math_stackers_offset_l\relax
     \egroup
   \fi}

\def\math_stackers_double_set_bottomtext
  {\edef\math_stackers_bottomtext{\mathstackersparameter\c!bottom}%
   \ifempty\math_stackers_bottomtext\else
     \global\setbox\b_math_stackers_bottom_text\hbox to \scratchwidth\bgroup
       \usemathstackersstyleandcolor\c!bottomstyle\c!bottomcolor
       \hskip\d_math_stackers_offset_l\relax
       \spac_align_simple{\mathstackersparameter\c!bottomalign}\math_stackers_bottomtext
     \egroup
   \fi}

\def\math_stackers_double_flush_texts
  {\ifvoid\b_math_stackers_top_text\else
     \superscript{\box\b_math_stackers_top_text}%
   \fi
   \ifvoid\b_math_stackers_bottom_text\else
     \subscript{\box\b_math_stackers_bottom_text}%
   \fi}

\tolerant\protected\def\math_stackers_make_double#top#bottom#category#codepoint#codeextra#spacer[#S#settings]#:#*#text%
  {\math_stackers_start_group{#category}%
   \c_math_stackers_top      #top\relax
   \c_math_stackers_bottom   #bottom\relax
   \c_math_stackers_codepoint#codepoint\relax
   \c_math_stackers_extracode#codeextra\relax
   \ifparameter#settings\or
     \setupcurrentmathstackers[#settings]%
   \fi
   \mathstackersparameter\c!left\relax
   \dostarttagged\t!mstacker\currentmathstackers
   \ifmmode\math_atom_by_parameter\mathstackersparameter\else\dontleavehmode\fi
     {\math_stackers_reset_variables
      \edef\m_math_stackers_text_middle{#text}%
      \math_stackers_check_unicode\c_math_stackers_codepoint
      \let\math_stackers_unicode_two\math_stackers_unicode
      \let\math_stackers_unicode_one\empty
      \ifempty\math_stackers_middle
        \setbox\scratchboxthree\emptyhbox
      \else
        \dostarttagged\t!mstackermid\empty
        \setmathtextbox\scratchboxthree\hbox{\math_stackers_middletext}%
        \dostoptagged
      \fi
      \scratchwidth\wd\scratchboxthree
      %
      \scratchdimen{\mathstackersparameter\c!minwidth}%
      \ifdim\scratchwidth<\scratchdimen
         \scratchwidth\scratchdimen
      \fi
      \advanceby\scratchwidth2\scratchhoffset
      %
     %\scratchunicode\c_math_stackers_codepoint
      \ifcase\c_math_stackers_bottom
        \d_math_stackers_offset_l\mathstackersparameter{lt}%
        \d_math_stackers_offset_r\mathstackersparameter{rt}%
      \orelse\ifcase\c_math_stackers_top
        \d_math_stackers_offset_l\mathstackersparameter{lb}%
        \d_math_stackers_offset_r\mathstackersparameter{rb}%
      \else
        \d_math_stackers_offset_l\mathstackersparameter{lt}%
        \d_math_stackers_offset_r\mathstackersparameter{rt}% or rb ?
      \fi
      %
      \setbox\scratchboxtwo\csname\??mathstackersalternative\p_alternative\endcsname
      \setbox\scratchboxthree\hpack to \scratchwidth{\hss\box\scratchboxthree\hss}%
      %
      \scratchunicode\c_math_stackers_extracode
      \ifcase\scratchunicode\else % uses \scratchunicode
        \d_math_stackers_offset_l\mathstackersparameter{lb}%
        \d_math_stackers_offset_r\mathstackersparameter{rb}%
        \setbox\scratchboxone\csname\??mathstackersalternative\p_alternative\endcsname
        \math_stackers_check_unicode\c_math_stackers_extracode
        \let\math_stackers_unicode_one\math_stackers_unicode
      \fi
      %
      \math_stackers_normalize_three
      \math_stackers_get_max_width
      \math_stackers_set_max_width
      % analysis: is this still ok here?
      \ifcase\c_math_stackers_bottom
        \ifcase\c_math_stackers_top
            \dosettagproperty\s!subtype\t!munderover
        \else
            \dosettagproperty\s!subtype\t!mover
        \fi
      \else
        \ifcase\c_math_stackers_top
            \dosettagproperty\s!subtype\t!munder
        \else
            % brrr
        \fi
      \fi
      % base
    % \math_stackers_start_tagged_mid\math_stackers_unicode
        \math_stackers_middle\bgroup
          \box\scratchboxthree
        \egroup
    % \math_stackers_stop_tagged
      %
\math_stackers_double_reset_texts
      %
      \ifdim\htdp\scratchboxtwo>\zeropoint
        \ifcase\c_math_stackers_bottom\else
          \kern-\scratchwidth
          % under
          \math_stackers_start_tagged_bot{\ifcase\c_math_stackers_top\math_stackers_unicode_two\else\math_stackers_unicode_one\fi}%
            \math_stackers_bottom\bgroup
              \lower{%
                 \scratchdepth
                +\ht\ifcase\c_math_stackers_top\scratchboxtwo\else\scratchboxone\fi
                +(\mathstackersparameter\c!distance)% was \c!voffset
              }\box\ifcase\c_math_stackers_top\scratchboxtwo\else\scratchboxone\fi
            \egroup
          \math_stackers_stop_tagged
\math_stackers_double_set_toptext
        \fi
        \ifcase\c_math_stackers_top\else
          \kern-\scratchwidth
          % over
          \math_stackers_start_tagged_top\math_stackers_unicode_two
            \math_stackers_top\bgroup
              \raise{%
                 \scratchheight
                +\dp\scratchboxtwo % new
                +(\mathstackersparameter\c!distance)% was \c!voffset
              }\box\scratchboxtwo
            \egroup
          \math_stackers_stop_tagged
\math_stackers_double_set_bottomtext
        \fi
      \fi}%
  \dostoptagged
  \mathstackersparameter\c!right\relax
  \math_stackers_stop_group
\math_stackers_double_flush_texts
   }

\permanent\tolerant\protected\def\definemathoverextensible[#1]#*[#2]#*[#3]%
  {\ifparameter#3\or
     \frozen\protected\edefcsname#2\endcsname{\math_stackers_make_double\plusone  \zerocount{#1}{\number#3}{0}}%
   \else
     \frozen\protected\edefcsname#1\endcsname{\math_stackers_make_double\plusone  \zerocount\noexpand\currentmathstackers{\number#2}{0}}%
   \fi}

\permanent\tolerant\protected\def\definemathunderextensible[#1]#*[#2]#*[#3]%
  {\ifparameter#3\or
     \frozen\protected\edefcsname#2\endcsname{\math_stackers_make_double\zerocount\plusone{#1}{\number#3}{0}}%
   \else
     \frozen\protected\edefcsname#1\endcsname{\math_stackers_make_double\zerocount\plusone\noexpand\currentmathstackers{\number#2}{0}}%
   \fi}

\permanent\tolerant\protected\def\definemathdoubleextensible[#1]#*[#2]#*[#3]#*[#4]%
  {\ifparameter#4\or
     \frozen\protected\edefcsname#2\endcsname{\math_stackers_make_double\plusone  \plusone{#1}{\number#3}{\number#4}}%
   \else
     \frozen\protected\edefcsname#1\endcsname{\math_stackers_make_double\plusone  \plusone\noexpand\currentmathstackers{\number#2}{\number#3}}%
   \fi}

\permanent\tolerant\protected\def\definemathover[#category]#spacer[#command]#spacer[#topcode]%
  {\frozen\protected\defcsname#command\endcsname{\math_stackers_handle_direct\plusone\zerocount{#category}{#topcode}{0}}}

\permanent\tolerant\protected\def\definemathunder[#category]#spacer[#command]#spacer[#bottomcode]%
  {\frozen\protected\defcsname#command\endcsname{\math_stackers_handle_direct\zerocount\plusone{#category}{#bottomcode}{0}}}

\permanent\tolerant\protected\def\definemathdouble[#category]#spacer[#command]#spacer[#topcode]#spacer[#bottomcode]%
  {\frozen\protected\defcsname#command\endcsname{\math_stackers_handle_direct\plusone\plusone{#category}{#topcode}{#bottomcode}}}

\permanent\tolerant\protected\def\mathover[#category]#spacer[#S#settings]#:#*#topcode#*#text%
  {\begingroup
   \cdef\currentmathstackers{\ifparameter#category\or#category\else\v!top\fi}%
   \ifparameter#settings\or
      \setupcurrentmathstackers[#settings]%
   \fi
   \math_stackers_make_double\plusone\zerocount
     {\currentmathstackers}%
     {#topcode}%
     {0}%
     {#text}%
   \endgroup}

\permanent\tolerant\protected\def\mathunder[#category]#spacer[#S#settings]#:#*#bottomcode#*#text%
  {\begingroup
   \cdef\currentmathstackers{\ifparameter#category\or#category\else\v!bottom\fi}%
   \ifparameter#settings\or
      \setupcurrentmathstackers[#settings]%
   \fi
   \math_stackers_make_double\zerocount\plusone
     {\currentmathstackers}%
     {#bottomcode}%
     {0}%
     {#text}%
   \endgroup}

\permanent\tolerant\protected\def\mathdouble[#category]#spacer[#settings]#:#*#topcode#*#bottomcode#*#text%
  {\begingroup
   \cdef\currentmathstackers{\ifparameter#category\or#category\else\v!both\fi}%
   \ifparameter#settings\or
      \setupcurrentmathstackers[#settings]%
   \fi
   \math_stackers_make_double\plusone\plusone
     {\currentmathstackers}%
     {#topcode}%
     {#bottomcode}%
     {#text}%
   \endgroup}

\def\math_stackers_handle_direct#top#bottom#category#topcode#bottomcode#text%
  {\begingroup
   \math_stackers_make_double#top#bottom{#category}{#topcode}{#bottomcode}{#text}%
   \endgroup}

%D A relative new one is a combination of accents and text (as needed in mathml):

\protected\def\math_stackers_make_double_text#where#category#codepoint#text#extra%
  {\math_stackers_start_group{#category}%
   \mathstackersparameter\c!left\relax
   \dostarttagged\t!mstacker\currentmathstackers
   \ifmmode\math_atom_by_parameter\mathstackersparameter\else\dontleavehmode\fi
     {\math_stackers_reset_variables
      \edef\m_math_stackers_text_middle{#text}%
      \math_stackers_check_unicode{#codepoint}%
      \scratchunicode#codepoint\relax
      %
      \ifempty\math_stackers_middle
        \setbox\scratchboxthree\emptyhbox
      \else
        \dostarttagged\t!mstackermid{\math_stackers_unicode}%
        \setmathtextbox\scratchboxthree\hbox{\math_stackers_middletext}%
        \dostoptagged
      \fi
      %
      \ifcase#where\relax
        \edef\m_math_stackers_text_top{#extra}%
        \ifempty\math_stackers_top
          \setbox\scratchboxone\emptyhbox
        \else
          \setmathsmalltextbox\scratchboxone\hbox{\math_stackers_toptext}%
        \fi
      \else
        \edef\m_math_stackers_text_bottom{#extra}%
        \ifempty\math_stackers_bottom
          \setbox\scratchboxone\emptyhbox
        \else
          \setmathsmalltextbox\scratchboxone\hbox{\math_stackers_bottomtext}%
        \fi
      \fi
      %
      \scratchwidth\wd
        \ifdim\wd\scratchboxone>\wd\scratchboxthree
          \scratchboxone
        \else
          \scratchboxthree
        \fi
      \relax
      \scratchdimen\mathstackersparameter\c!minwidth\relax
      \ifdim\scratchwidth<\scratchdimen
         \scratchwidth\scratchdimen
      \fi
      \advanceby\scratchwidth2\scratchhoffset
      %
      \ifdim\wd\scratchboxone<\scratchwidth
        \setbox\scratchboxone\hpack to \scratchwidth{\hss\unhbox\scratchboxone\hss}%
      \fi
      \ifdim\wd\scratchboxthree<\scratchwidth
        \setbox\scratchboxthree\hpack to \scratchwidth{\hss\unhbox\scratchboxthree\hss}%
      \fi
      %
      \math_stackers_normalize_three
      % analysis: is this still ok here?
      \dosettagproperty\s!subtype\t!munderover
      % base
      \math_stackers_middle\bgroup
        \box\scratchboxthree
      \egroup
      %
      \setbox\scratchboxtwo\csname\??mathstackersalternative\p_alternative\endcsname
      \kern-\scratchwidth
      \ifcase#where\relax
        % under
        \math_stackers_start_tagged_bot\empty
          \math_stackers_bottom\bgroup
            \lower{%
              \scratchdepth
             +\ht\scratchboxtwo
             +(\mathstackersparameter\c!distance)%
            }\box\scratchboxtwo % accent
          \egroup
        \math_stackers_stop_tagged
        \kern-\scratchwidth
        % over
        \math_stackers_start_tagged_top\empty
          \math_stackers_top\bgroup
            \raise{%
               \scratchheight
              +\dp\scratchboxone
              +(\mathstackersparameter\c!voffset)%
            }\box\scratchboxone % toptext
          \egroup
        \math_stackers_stop_tagged
      \else
        % under
        \math_stackers_start_tagged_bot\empty
          \math_stackers_bottom\bgroup
            \lower{%
               \scratchdepth
              +\ht\scratchboxone
              +(\mathstackersparameter\c!voffset)%
            }\box\scratchboxone % bottext
          \egroup
        \math_stackers_stop_tagged
        \kern-\scratchwidth
        % over
        \math_stackers_start_tagged_top\empty
          \math_stackers_top\bgroup
            \raise{%
               \scratchheight
              +\dp\scratchboxtwo % new
              +(\mathstackersparameter\c!distance)%
            }\box\scratchboxtwo % accent
          \egroup
        \math_stackers_stop_tagged
      \fi
      }%
   \dostoptagged
   \mathstackersparameter\c!right\relax
   \math_stackers_stop_group}

\permanent\tolerant\protected\def\definemathovertextextensible[#1]#*[#2]#*[#3]%
  {\ifparameter#3\or
     \frozen\protected\edefcsname#2\endcsname{\math_stackers_make_double_text\plusone{#1}{\number#3}}%
   \else
     \frozen\protected\edefcsname#1\endcsname{\math_stackers_make_double_text\plusone\noexpand\currentmathstackers{\number#2}}%
   \fi}

\permanent\tolerant\protected\def\definemathundertextextensible[#1]#*[#2]#*[#3]%
  {\ifparameter#3\or
     \frozen\protected\edefcsname#2\endcsname{\math_stackers_make_double_text\zerocount{#1}{\number#3}}%
   \else
     \frozen\protected\edefcsname#1\endcsname{\math_stackers_make_double_text\zerocount\noexpand\currentmathstackers{\number#2}}%
   \fi}

\permanent\tolerant\protected\def\mathovertext[#category]%
  {\begingroup
   \math_stackers_direct_double_text\plusone  {\ifarguments#category\or#category\else\v!top\fi}}

\permanent\tolerant\protected\def\mathundertext[#category]%
  {\begingroup
   \math_stackers_direct_double_text\zerocount{\ifarguments#category\or#category\else\v!bottom\fi}}

\def\math_stackers_direct_double_text#where#category#codepoint#text#extra%%
  {\math_stackers_make_double_text#where{#category}{#codepoint}{#text}{#extra}%
   \endgroup}

%D Here is a bonus macro that takes three texts. It can be used to get consistent
%D mixed usage.

\permanent\tolerant\protected\def\mathtriplet[#1]#:#*#=#*#=#*#=%
  {\begingroup
   \ifparameter#1\or\cdef\currentmathstackers{#1}\fi
   \math_stackers_triplet\plusone\currentmathstackers{#2}{#3}{#4}%
   \endgroup}

\permanent\tolerant\protected\def\definemathtriplet[#1]#*[#2]#*[#3]% category name default
  {\ifarguments\or
     \frozen\protected\edefcsname#1\endcsname{\math_stackers_auto_triplet_nop[\noexpand\currentmathstackers]}%
   \or
     \frozen\protected\edefcsname#2\endcsname{\math_stackers_auto_triplet_nop[#1]}%
   \or
     \frozen\protected\edefcsname#2\endcsname{\math_stackers_auto_triplet_yes[#1][#3]}%
   \fi}

\tolerant\protected\def\math_stackers_auto_triplet_yes[#1][#2]#*[#3]#:#*#=#*#=% [#2]% #2 gobble spaces
  {\begingroup
   \cdef\currentmathstackers{#1}%
   \def \m_math_stackers_text_middle{#2}%
   \ifparameter#3\or\cdef\currentmathstackers{#3}\fi
   \math_stackers_triplet\plusone\currentmathstackers\m_math_stackers_text_middle{#4}{#5}%
   \endgroup}

\tolerant\protected\def\math_stackers_auto_triplet_nop[#1]#*[#2]#:#*#=#*#=#*#=% [#2]% #2 gobble spaces%
  {\begingroup
   \cdef\currentmathstackers{#1}%
   \ifparameter#2\or\cdef\currentmathstackers{#2}\fi
   \math_stackers_triplet\plusone\currentmathstackers{#3}{#4}{#5}%
   \endgroup}

%D Definitions:

\definemathstackers
  [\v!mathematics]
  [\c!topcommand=\mathematics,
   \c!middlecommand=\mathematics,
   \c!bottomcommand=\mathematics]

\definemathstackers
  [\s!math]
  [\v!mathematics]

\definemathstackers
  [\v!text]
  [\v!mathematics]
  [\c!topcommand=,
   \c!middlecommand=\mathematics,
   \c!bottomcommand=]

\definemathstackers
  [\v!reverse]
  [\v!mathematics]
  [\c!order=\v!reverse]

\definemathstackers
  [\v!both]
  [\v!mathematics]
  [\c!location=\v!top, % ?
   \c!mathclass=\s!accent,  % check chemistry
   \c!strut=\v!no,
   \c!middlecommand=\mathematics,
   \c!hoffset=\zeropoint]

\definemathstackers
  [\v!top]
  [\v!both]

\definemathstackers
  [\v!bottom]
  [\v!both]

\definemathstackers
  [\v!vfenced]
  [\v!both]
  [\c!mathclass=\s!ordinary,
   \c!mathlimits=\v!yes,
   % only these arrows make sense
   \c!stretch=\v!yes,
   \c!shrink=\v!yes]

% these are needed for mathml:

% \setupmathstackers
%   [\v!both]
%   [\c!hoffset=1pt,
%    \c!voffset=1pt]

\definemathstackers
  [\v!bothtext]
  [\v!both]
  [\c!strut=\v!yes]

% These are compatibity definitions, math only.

% todo: top= bottom= middle= is nicer (compare math-fen)

%D We save a few definitions that we automatically got from the \type {char-def.lua}
%D database.

% Be careful in choosing what accents you take (the code below uses a combining
% one):
%
% \startbuffer
% % $\Umathaccent top        0 0 "20D7 {example}$
% % $\Umathaccent top  fixed 0 0 "20D7 {example}$
% $\Umathaccent              0 0 "20D7 {example}$
% $\Umathaccent        fixed 0 0 "20D7 {example}$
% $\Umathaccent bottom       0 0 "20D7 {example}$
% $\Umathaccent bottom fixed 0 0 "20D7 {example}$
% $\Umathaccent both         0 0 "20D7
%                            0 0 "20D7 {example}$
% $\Umathaccent both fixed   0 0 "20D7
%                    fixed   0 0 "20D7 {example}$
% $\Umathaccent both         0 0 "20D7
%                    fixed   0 0 "20D7 {example}$
% $\Umathaccent both fixed   0 0 "20D7
%                            0 0 "20D7 {example}$
% \stopbuffer
%
% \setupbodyfont[modern]  \getbuffer
% \setupbodyfont[xits]    \getbuffer
% \setupbodyfont[cambria] \getbuffer

\permanent\protected\def\normaldoublebrace {\Umathaccent \s!both \zerocount \zerocount "23DE \zerocount \zerocount "23DF }
\permanent\protected\def\normaldoubleparent{\Umathaccent \s!both \zerocount \zerocount "23DC \zerocount \zerocount "23DD }

% let's keep this .. some are not defined so there's nothing normal

\aliased\let\normaloverbrace      \overbrace
\aliased\let\normalunderbrace     \underbrace
\aliased\let\normaloverparent     \overparent
\aliased\let\normalunderparent    \underparent
\aliased\let\normaloverbracket    \overbracket
\aliased\let\normalunderbracket   \underbracket

%aliased\let\normalunderleftarrow \underleftarrow
%aliased\let\normaloverleftarrow  \overleftarrow
%aliased\let\normalunderrightarrow\underrightarrow
%aliased\let\normaloverrightarrow \overrightarrow

%D Here come the new ones:

\definemathstackers [\v!none]   [\v!mathematics] [\c!hoffset=\zeropoint]
\definemathstackers [\v!normal] [\v!mathematics] [\c!hoffset=0.5mq] % the default
\definemathstackers [\v!small]  [\v!mathematics] [\c!hoffset=1mq]
\definemathstackers [\v!medium] [\v!mathematics] [\c!hoffset=1.5mq]
\definemathstackers [\v!big]    [\v!mathematics] [\c!hoffset=2mq]

\definemathextensible [\v!reverse] [xrel]                ["2212] % ["002D]
\definemathextensible [\v!reverse] [xequal]              ["003D]
\definemathextensible [\v!reverse] [xleftarrow]          ["2190] % ["27F5]
\definemathextensible [\v!reverse] [xrightarrow]         ["2192] % ["27F6]
\definemathextensible [\v!reverse] [xleftrightarrow]     ["2194] % ["27F7]
\definemathextensible [\v!reverse] [xlongleftarrow]      ["27F5]
\definemathextensible [\v!reverse] [xlongrightarrow]     ["27F6]
\definemathextensible [\v!reverse] [xlongleftrightarrow] ["27F7]
\definemathextensible [\v!reverse] [xLeftarrow]          ["27F8] % why not ["27D2]
\definemathextensible [\v!reverse] [xRightarrow]         ["27F9] % why not ["27D0]
\definemathextensible [\v!reverse] [xLeftrightarrow]     ["27FA] % why not ["27D4]
\definemathextensible [\v!reverse] [xtwoheadleftarrow]   ["219E]
\definemathextensible [\v!reverse] [xtwoheadrightarrow]  ["21A0]
\definemathextensible [\v!reverse] [xmapsto]             ["21A6]
\definemathextensible [\v!reverse] [xhookleftarrow]      ["21A9]
\definemathextensible [\v!reverse] [xhookrightarrow]     ["21AA]
\definemathextensible [\v!reverse] [xleftharpoondown]    ["21BD]
\definemathextensible [\v!reverse] [xleftharpoonup]      ["21BC]
\definemathextensible [\v!reverse] [xrightharpoondown]   ["21C1]
\definemathextensible [\v!reverse] [xrightharpoonup]     ["21C0]
\definemathextensible [\v!reverse] [xrightoverleftarrow] ["21C4]
\definemathextensible [\v!reverse] [xleftoverrightarrow] ["21C6]
\definemathextensible [\v!reverse] [xleftrightharpoons]  ["21CB]
\definemathextensible [\v!reverse] [xrightleftharpoons]  ["21CC]
\definemathextensible [\v!reverse] [xtriplerel]          ["2261]

\definemathextensible [\v!mathematics] [mrel]                ["2212] % ["002D]
\definemathextensible [\v!mathematics] [mequal]              ["003D]
\definemathextensible [\v!mathematics] [mleftarrow]          ["2190] % ["27F5]
\definemathextensible [\v!mathematics] [mrightarrow]         ["2192] % ["27F6]
\definemathextensible [\v!mathematics] [mleftrightarrow]     ["2194] % ["27F7]
\definemathextensible [\v!mathematics] [mLeftarrow]          ["21D0]
\definemathextensible [\v!mathematics] [mRightarrow]         ["21D2]
\definemathextensible [\v!mathematics] [mLeftrightarrow]     ["21D4]
\definemathextensible [\v!mathematics] [mtwoheadleftarrow]   ["219E]
\definemathextensible [\v!mathematics] [mtwoheadrightarrow]  ["21A0]
\definemathextensible [\v!mathematics] [mmapsto]             ["21A6]
\definemathextensible [\v!mathematics] [mhookleftarrow]      ["21A9]
\definemathextensible [\v!mathematics] [mhookrightarrow]     ["21AA]
\definemathextensible [\v!mathematics] [mleftharpoondown]    ["21BD]
\definemathextensible [\v!mathematics] [mleftharpoonup]      ["21BC]
\definemathextensible [\v!mathematics] [mrightharpoondown]   ["21C1]
\definemathextensible [\v!mathematics] [mrightharpoonup]     ["21C0]
\definemathextensible [\v!mathematics] [mrightoverleftarrow] ["21C4]
\definemathextensible [\v!mathematics] [mleftoverrightarrow] ["21C6]
\definemathextensible [\v!mathematics] [mleftrightharpoons]  ["21CB]
\definemathextensible [\v!mathematics] [mrightleftharpoons]  ["21CC]
\definemathextensible [\v!mathematics] [mtriplerel]          ["2261]

\definemathextensible [\v!text] [trel]                ["2212] % ["002D]
\definemathextensible [\v!text] [tequal]              ["003D]
\definemathextensible [\v!text] [tmapsto]             ["21A6]
\definemathextensible [\v!text] [tleftarrow]          ["2190] % ["27F5]
\definemathextensible [\v!text] [trightarrow]         ["2192] % ["27F6]
\definemathextensible [\v!text] [tleftrightarrow]     ["2194] % ["27F7]
\definemathextensible [\v!text] [tLeftarrow]          ["21D0]
\definemathextensible [\v!text] [tRightarrow]         ["21D2]
\definemathextensible [\v!text] [tLeftrightarrow]     ["21D4]
\definemathextensible [\v!text] [ttwoheadleftarrow]   ["219E]
\definemathextensible [\v!text] [ttwoheadrightarrow]  ["21A0]
\definemathextensible [\v!text] [tmapsto]             ["21A6]
\definemathextensible [\v!text] [thookleftarrow]      ["21A9]
\definemathextensible [\v!text] [thookrightarrow]     ["21AA]
\definemathextensible [\v!text] [tleftharpoondown]    ["21BD]
\definemathextensible [\v!text] [tleftharpoonup]      ["21BC]
\definemathextensible [\v!text] [trightharpoondown]   ["21C1]
\definemathextensible [\v!text] [trightharpoonup]     ["21C0]
\definemathextensible [\v!text] [trightoverleftarrow] ["21C4]
\definemathextensible [\v!text] [tleftoverrightarrow] ["21C6]
\definemathextensible [\v!text] [tleftrightharpoons]  ["21CB]
\definemathextensible [\v!text] [trightleftharpoons]  ["21CC]
\definemathextensible [\v!text] [ttriplerel]          ["2261]

\definemathoverextensible [\v!top] [overleftarrow]          ["2190] % ["27F5]
\definemathoverextensible [\v!top] [overrightarrow]         ["2192] % ["27F6]
\definemathoverextensible [\v!top] [overleftrightarrow]     ["2194] % ["27F7]
\definemathoverextensible [\v!top] [overtwoheadleftarrow]   ["219E] % ["27F8]
\definemathoverextensible [\v!top] [overtwoheadrightarrow]  ["21A0] % ["27F9]
\definemathoverextensible [\v!top] [overlefttailarrow]      ["21A2]
\definemathoverextensible [\v!top] [overrighttailarrow]     ["21A3]
\definemathoverextensible [\v!top] [overleftbararrow]       ["21A4]
\definemathoverextensible [\v!top] [overrightbararrow]      ["21A6]
\definemathoverextensible [\v!top] [overlefthookarrow]      ["21A9]
\definemathoverextensible [\v!top] [overrighthookarrow]     ["21AA]
\definemathoverextensible [\v!top] [overleftharpoondown]    ["21BD]
\definemathoverextensible [\v!top] [overleftharpoonup]      ["21BC]
\definemathoverextensible [\v!top] [overrightharpoondown]   ["21C1]
\definemathoverextensible [\v!top] [overrightharpoonup]     ["21C0]
\definemathoverextensible [\v!top] [overRightarrow]         ["21D2] % ["27F9]
\definemathoverextensible [\v!top] [overLeftarrow]          ["21D0] % ["27F8]
\definemathoverextensible [\v!top] [overLeftrightarrow]     ["21D4] % ["27F8]
\definemathoverextensible [\v!top] [overLeftbararrow]       ["2906]
\definemathoverextensible [\v!top] [overRightbararrow]      ["2907]

\definemathunderextensible [\v!bottom] [underleftarrow]         ["2190] % ["27F5]
\definemathunderextensible [\v!bottom] [underrightarrow]        ["2192] % ["27F6]
\definemathunderextensible [\v!bottom] [underleftrightarrow]    ["2194] % ["27F7]
\definemathunderextensible [\v!bottom] [undertwoheadleftarrow]  ["219E] % ["27F8]
\definemathunderextensible [\v!bottom] [undertwoheadrightarrow] ["21A0] % ["27F9]
\definemathunderextensible [\v!bottom] [underlefttailarrow]     ["21A2]
\definemathunderextensible [\v!bottom] [underrighttailarrow]    ["21A3]
\definemathunderextensible [\v!bottom] [underleftbararrow]      ["21A4]
\definemathunderextensible [\v!bottom] [underrightbararrow]     ["21A6]
\definemathunderextensible [\v!bottom] [underlefthookarrow]     ["21A9]
\definemathunderextensible [\v!bottom] [underrighthookarrow]    ["21AA]
\definemathunderextensible [\v!bottom] [underleftharpoondown]   ["21BD]
\definemathunderextensible [\v!bottom] [underleftharpoonup]     ["21BC]
\definemathunderextensible [\v!bottom] [underrightharpoondown]  ["21C1]
\definemathunderextensible [\v!bottom] [underrightharpoonup]    ["21C0]
\definemathunderextensible [\v!bottom] [underRightarrow]        ["21D2] % ["27F9]
\definemathunderextensible [\v!bottom] [underLeftarrow]         ["21D0] % ["27F8]
\definemathunderextensible [\v!bottom] [underLeftrightarrow]    ["21D4] % ["27F8]
\definemathunderextensible [\v!bottom] [underLeftbararrow]      ["2906]
\definemathunderextensible [\v!bottom] [underRightbararrow]     ["2907]

%D We don't use overline and underline. This is one of the overlooked aspects of
%D unicode cq. opentype math: why treat rules different than e.g. arrows and
%D accents. It is a bit unfortunate that the opportunity to move math to new
%D technologies happened outside the tex domain (and/or some aspects were kept while
%D in fact they were side effects of limitations of traditional fonts). From the
%D unicode aware tex engines' implementation point of view things could have been
%D done a bit nicer but then: the community didn't seem to care too much and just
%D has to follow now.
%D
%D Anyhow, we use a character based approach so that at least we get unicode stuff
%D in the backend (okay, we still need to deal with some cut and paste issues but at
%D least we now know what we deal with.

\definemathoverextensible   [\v!vfenced] [overbar]       ["203E]         % todo: private
\definemathunderextensible  [\v!vfenced] [underbar]              ["203E] % todo: private
\definemathdoubleextensible [\v!vfenced] [doublebar]     ["203E] ["203E] % todo: private

\definemathoverextensible   [\v!vfenced] [overbrace]     ["23DE]
\definemathunderextensible  [\v!vfenced] [underbrace]            ["23DF]
\definemathdoubleextensible [\v!vfenced] [doublebrace]   ["23DE] ["23DF]

\definemathoverextensible   [\v!vfenced] [overparent]    ["23DC]
\definemathunderextensible  [\v!vfenced] [underparent]           ["23DD]
\definemathdoubleextensible [\v!vfenced] [doubleparent]  ["23DC] ["23DD]

\definemathoverextensible   [\v!vfenced] [overbracket]   ["23B4]
\definemathunderextensible  [\v!vfenced] [underbracket]          ["23B5]
\definemathdoubleextensible [\v!vfenced] [doublebracket] ["23B4] ["23B5]

%D For mathml:

\definemathdoubleextensible    [\v!both]     [overbarunderbar]         ["203E] ["203E] % todo: private
\definemathdoubleextensible    [\v!both]     [overbraceunderbrace]     ["23DE] ["23DF]
\definemathdoubleextensible    [\v!both]     [overparentunderparent]   ["23DC] ["23DD]
\definemathdoubleextensible    [\v!both]     [overbracketunderbracket] ["23B4] ["23B5]

\definemathovertextextensible  [\v!bothtext] [overbartext]             ["203E]         % todo: private
\definemathundertextextensible [\v!bothtext] [underbartext]                    ["203E] % todo: private
\definemathovertextextensible  [\v!bothtext] [overbracetext]           ["23DE]
\definemathundertextextensible [\v!bothtext] [underbracetext]                  ["23DF]
\definemathovertextextensible  [\v!bothtext] [overparenttext]          ["23DC]
\definemathundertextextensible [\v!bothtext] [underparenttext]                 ["23DD]
\definemathovertextextensible  [\v!bothtext] [overbrackettext]         ["23B4]
\definemathundertextextensible [\v!bothtext] [underbrackettext]                ["23B5]

% \def\math_stackers_hacked_fill#1#2#3%
%   {\mathematics
%      {\begingroup
%       \mathsurround\zeropoint
%       \thickmuskip \zeromuskip
%       \medmuskip   \zeromuskip
%       \thinmuskip  \zeromuskip
%       \tinymuskip  \zeromuskip
%       \pettymuskip \zeromuskip
%       \ifrelax#1%
%         \cleaders\mathstylehbox{#2}\hfill
%       \else
%         #1%
%         \mkern-7\onemuskip
%         \cleaders\mathstylehbox{\mkern-2\onemuskip#2\mkern-2\onemuskip}\hfill
%         \mkern-7\onemuskip
%         #3%
%       \fi
%       \endgroup}}
%
% \permanent\protected\def\rightarrowfill        {\math_stackers_hacked_fill \relbar               \relbar              \rightarrow}
% \permanent\protected\def\leftarrowfill         {\math_stackers_hacked_fill \leftarrow            \relbar              \relbar}
% \permanent\protected\def\rightoverleftarrowfill{\math_stackers_hacked_fill \relax                \crightoverleftarrow \relax}
% \permanent\protected\def\leftoverrightarrowfill{\math_stackers_hacked_fill \relax                \cleftoverrightarrow \relax}
% \permanent\protected\def\equalfill             {\math_stackers_hacked_fill \Relbar               \Relbar              \Relbar}
% \permanent\protected\def\Rightarrowfill        {\math_stackers_hacked_fill \Relbar               \Relbar              \Rightarrow}
% \permanent\protected\def\Leftarrowfill         {\math_stackers_hacked_fill \Leftarrow            \Relbar              \Relbar}
% \permanent\protected\def\Leftrightarrowfill    {\math_stackers_hacked_fill \Leftarrow            \Relbar              \Rightarrow}
% \permanent\protected\def\leftrightarrowfill    {\math_stackers_hacked_fill \leftarrow            \relbar              \rightarrow}
% \permanent\protected\def\mapstofill            {\math_stackers_hacked_fill{\mapstochar\relbar}   \relbar              \rightarrow}
% \permanent\protected\def\twoheadrightarrowfill {\math_stackers_hacked_fill \relbar               \relbar              \twoheadrightarrow}
% \permanent\protected\def\twoheadleftarrowfill  {\math_stackers_hacked_fill \twoheadleftarrow     \relbar              \relbar}
% \permanent\protected\def\rightharpoondownfill  {\math_stackers_hacked_fill \relbar               \relbar              \rightharpoondown}
% \permanent\protected\def\rightharpoonupfill    {\math_stackers_hacked_fill \relbar               \relbar              \rightharpoonup}
% \permanent\protected\def\leftharpoondownfill   {\math_stackers_hacked_fill \leftharpoondown      \relbar              \relbar}
% \permanent\protected\def\leftharpoonupfill     {\math_stackers_hacked_fill \leftharpoonup        \relbar              \relbar}
% \permanent\protected\def\hookleftfill          {\math_stackers_hacked_fill \leftarrow            \relbar             {\relbar\joinrel\rhook}}
% \permanent\protected\def\hookrightfill         {\math_stackers_hacked_fill{\lhook\joinrel\relbar}\relbar              \rightarrow}
% \permanent\protected\def\relfill               {\math_stackers_hacked_fill \relbar               \relbar              \relbar}
% \permanent\protected\def\triplerelfill         {\math_stackers_hacked_fill \equiv                \equiv               \equiv}
%
% \permanent\tolerant\protected\def\defineextensiblefiller[#1]#*[#2]%
%   {\permanent\letcsname\??mathextensiblefallbacks\number#2\expandafter\endcsname\csname#1\endcsname}

\installcorenamespace {mathadaptiveextensible}

\defineadaptive
  [mathfiller]
  [\c!setups=adaptive:mathfiller,
   \c!stretch=1fill]

\startsetups adaptive:mathfiller
    \setbox\usedadaptivebox\hbox to \usedadaptivewidth \bgroup
        \startimath
            \Uhextensible
                \s!width  \usedadaptivewidth
                \s!middle \zerocount \usedadaptivealternative
            \relax
        \stopimath
    \egroup
\stopsetups

% \def\mathfiller#1%
%   {\begingroup
%    \scratchunicode#1\relax
%    \adaptivebox[mathfiller][\c!alternative=#1]{\hss\strut\hss}%
%    \endgroup}

\permanent\protected\def\mathfiller#1%
  {\adaptivebox[mathfiller][\c!alternative=#1]{\hss\strut\hss}}

\permanent\tolerant\protected\def\defineextensiblefiller[#1]#*[#2]%
  {\frozen\instance\edefcsname#1\endcsname{\mathfiller{\number#2}}}

%defineextensiblefiller [barfill]                ["203E] % % todo: private
\defineextensiblefiller [relfill]                ["2212] % ["002D]
\defineextensiblefiller [equalfill]              ["003D]
\defineextensiblefiller [leftarrowfill]          ["2190]
\defineextensiblefiller [rightarrowfill]         ["2192]
\defineextensiblefiller [twoheadleftarrowfill]   ["219E]
\defineextensiblefiller [twoheadrightarrowfill]  ["21A0]
\defineextensiblefiller [mapstofill]             ["21A6]
%defineextensiblefiller [hookleftarrowfill]      ["21A9] % not in fonts
%defineextensiblefiller [hookrightarrowfill]     ["21AA] % not in fonts
\defineextensiblefiller [leftharpoondownfill]    ["21BD]
\defineextensiblefiller [leftharpoonupfill]      ["21BC]
\defineextensiblefiller [rightharpoondownfill]   ["21C1]
\defineextensiblefiller [rightharpoonupfill]     ["21C0]
\defineextensiblefiller [rightoverleftarrowfill] ["21C4]
\defineextensiblefiller [leftoverrightarrowfill] ["21C6]
%defineextensiblefiller [leftrightharpoonsfill]  ["21CB] % yet undefined
%defineextensiblefiller [rightleftharpoonsfill]  ["21CC] % yet undefined
\defineextensiblefiller [triplerelfill]          ["2261]
\defineextensiblefiller [leftrightarrowfill]     ["27F7]
\defineextensiblefiller [Leftarrowfill]          ["27F8]
\defineextensiblefiller [Rightarrowfill]         ["27F9]
\defineextensiblefiller [Leftrightarrowfill]     ["27FA]
\defineextensiblefiller [Rightleftarrowfill]     ["27FA]

%D Extra:

\permanent\protected\edef\singlebond{\mathematics{\mathsurround\zeropoint\char\number"002D\relax}}
\permanent\protected\edef\doublebond{\mathematics{\mathsurround\zeropoint\char\number"003D\relax}}
\permanent\protected\edef\triplebond{\mathematics{\mathsurround\zeropoint\char\number"2261\relax}}

% \mathchardef\singlebond"002D
% \mathchardef\doublebond"003D
% \mathchardef\triplebond"2261

%D Also handy:

\permanent\tolerant\protected\def\definemathunstacked[#1]#*[#2]#*[#3]% category name unicode
  {\ifarguments\or\or
     \frozen\protected\edefcsname#1\endcsname{\math_stackers_unstacked_normal\noexpand\currentmathstackers{\number#2}}%
   \or
     \frozen\protected\edefcsname#2\endcsname{\math_stackers_unstacked_normal{#1}{\number#3}}%
   \fi}

\protected\def\math_stackers_unstacked_normal#category#codepoint%
  {\begingroup
   \cdef\currentmathstackers{#category}%
   \scratchdistance\zeropoint
   \scratchcounter\ifchknum\mathstackersparameter\c!sample\or\mathstackersparameter\c!sample\else\zerocount\fi
   \scratchunicode#codepoint\relax
   \scratchclass\mathcodechecked{\mathstackersparameter\c!mathclass}\relax
   \ifconditional\indisplaymath\ifcase\scratchcounter\orelse\ifnum\lastatomclass=\mathbegincode
     \scratchdistance{%
       \fontcharwd\mathstylefont\mathstyle\scratchcounter
      -\fontcharwd\mathstylefont\mathstyle\scratchunicode
     }%
   \fi\fi
   \math_atom_by_parameter\mathstackersparameter
     {\usemathstackerscolorparameter\c!color
      \Umathchar\scratchclass\fam\scratchunicode\relax
      \ifzeropt\scratchdistance\else
        \kern\scratchdistance
      \fi}%
   \endgroup}

\definemathstackers
  [\s!implication]
  [\c!mathclass=\s!implication,
   \c!sample="27FA]

\definemathunstacked [\s!implication] [impliedby] ["27F8]
\definemathunstacked [\s!implication] [implies]   ["27F9]
\definemathunstacked [\s!implication] [iff]       ["27FA] % \ifandonlyif
\definemathunstacked [\s!implication] [impliesby] ["27FA]

% This is a weird one:

\definemathstackers
  [\v!wide]
  [\c!mathclass=\s!implication]

\definemathunstacked [\v!wide] [And]["0026] % \mathrel{\;&\;}

% New (an example of using mx):

\definemathstackers
  [\v!symbol]
  [\c!voffset=-.3mx,
   \c!hoffset=\zeropoint,
   \c!mathclass=\s!ordinary,
   \c!topoffset=.4mq, % poor man's italic correction
   \c!middlecommand=\mathematics]

\definemathover[\v!symbol][interiorset]["2218]

\protect \endinput

% \mathrel{\mathop{\hbox to \dimen0{\hss\copy4\hss}}
% \limits\normalsuperscript{\box0}\normalsubscript{\box2}}%

% $\Uoverdelimiter \zerocount "2194 {xxxx}$
% $\Uunderdelimiter\zerocount "2194 {xxxx}$
% $\Udelimiterover \zerocount "2194 {xxxx}$
% $\Udelimiterunder\zerocount "2194 {xxxx}$
% $\Udelimiterover \zerocount "219A {\Udelimiterunder \zerocount "219B {xxxx}}$

% $a \mathrel{\mathop{\filledhboxr{mid}}}\limits^{\filledhboxg{\strut top}}_{\filledhboxb{\strut bottom}} b$