%% mathformule.sty
%% A small display-math environment with no explicit auxiliary-package load.
%%
%% Copyright (C) 2026 Quan Sun
%% Released under the LaTeX Project Public License, version 1.3c or later.
%% This work has LPPL maintenance status `maintained'.
%% The Current Maintainer is Quan Sun.

\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{mathformule}[2026/07/22 v0.6 Lightweight multiline formulas]

\makeatletter

% The amount by which the normal baseline separation is opened up between
% formula rows.  Three points follows TeX's traditional multiline-math
% default.  It is a dimension (not glue), so row baselines stay predictable.
\newdimen\mathformulelineskip
\mathformulelineskip=3pt

% Global correction to the normal row separation.  A lineskip key on an
% individual formule replaces this value, so local settings take precedence.
% The assignment is global by design: the public command is a document-wide
% setup interface, normally used in the preamble.
\newdimen\mathformule@globallineskipadjust
\mathformule@globallineskipadjust=0pt
\newcommand*\formuleskip[1]{%
  \global\mathformule@globallineskipadjust=#1\relax
}

% Package options only select a numbering parent.  With no option, the
% existing definition of the equation counter is left completely untouched.
\let\mathformule@numberwithin\@empty
\DeclareOption{section}{\def\mathformule@numberwithin{section}}
\DeclareOption{chapter}{\def\mathformule@numberwithin{chapter}}
\DeclareOption*{%
  \PackageWarning{mathformule}{Unknown option `\CurrentOption' ignored}%
}
\ProcessOptions\relax

% Bind the shared equation counter to any existing LaTeX counter.  Since the
% standard equation counter is changed, native equation environments and
% formule continue to share both their sequence and printed representation.
\newcommand*\mathformulenumberwithin[1]{%
  \@ifundefined{c@#1}{%
    \PackageError{mathformule}{Counter `#1' is not defined}{%
      Use the name of an existing LaTeX counter, for example `section'.}%
  }{%
    \ifx\mathformule@numberwithin\@empty
    \else
      \@removefromreset{equation}{\mathformule@numberwithin}%
    \fi
    \@addtoreset{equation}{#1}%
    \expandafter\gdef\expandafter\theequation\expandafter{%
      \csname the#1\endcsname.\arabic{equation}}%
    \def\mathformule@numberwithin{#1}%
  }%
}

\ifx\mathformule@numberwithin\@empty
\else
  \expandafter\mathformulenumberwithin\expandafter
    {\mathformule@numberwithin}%
\fi

\newdimen\mathformule@lineskip
\newdimen\mathformule@lineskipadjust
\newdimen\mathformule@lbraceadjust
\newdimen\mathformule@rbraceadjust
\newcount\mathformule@braceside
\newcount\mathformule@rowcount
\newcount\mathformule@rowindex
\newcount\mathformule@typesetrow
\newcount\mathformule@cellcount
\newcount\mathformule@scancells
\newif\ifmathformule@multnum
\newif\ifmathformule@rowtag
\newif\ifmathformule@fleqn
\newif\ifmathformule@toomanycols
\newif\ifmathformule@trailingbreak
\newif\ifmathformule@hasalignment

% A monotonically increasing counter gives every multnum row its own reference
% target.  Its printed representation is set locally to the visible subnumber;
% the underlying value is never reset, so hyperref destinations stay unique
% even when equation numbering restarts in a new section.
\newcounter{mathformulerow}
\providecommand*\mathformulerowautorefname{\equationautorefname}
\AtBeginDocument{%
  \@ifpackageloaded{cleveref}{%
    \crefalias{mathformulerow}{equation}%
  }{}%
}

\def\mathformule@key@lineskip{lineskip}
\def\mathformule@key@lbrace{lbrace}
\def\mathformule@key@rbrace{rbrace}
\def\mathformule@key@lbraceskip{lbraceskip}
\def\mathformule@key@rbraceskip{rbraceskip}
\def\mathformule@key@multnum{multnum}

\def\mathformule@style@aa@dot{arabic.arabic}
\def\mathformule@style@aa@dash{arabic-arabic}
\def\mathformule@style@ar@dot{arabic.roman}
\def\mathformule@style@ar@dash{arabic-roman}
\def\mathformule@style@arabic{arabic}
\def\mathformule@style@roman{roman}

% A deliberately small key--value reader for the environment option.  Commas
% inside a value are protected by braces in the usual TeX way.  Appending two
% equals signs lets the same splitter accept both a bare key and key={value}.
% A bare unrecognised item retains the prototype shorthand [<dimension>]
% and has the same signed-correction semantics as lineskip=<dimension>.
\def\mathformule@parseoptions#1{%
  \@for\mathformule@item:=#1\do{%
    \expandafter\mathformule@splititem\mathformule@item==\@nil
  }%
}

\def\mathformule@splititem#1=#2=#3\@nil{%
  \edef\mathformule@key{\zap@space#1 \@empty}%
  \def\mathformule@value{#2}%
  \ifx\mathformule@key\mathformule@key@lbrace
    \mathformule@setbrace{1}{#2}%
  \else\ifx\mathformule@key\mathformule@key@rbrace
    \mathformule@setbrace{2}{#2}%
  \else\ifx\mathformule@key\mathformule@key@multnum
    \mathformule@setmultnum{#2}%
  \else\ifx\mathformule@key\mathformule@key@lineskip
    \ifx\mathformule@value\@empty
      \PackageError{mathformule}{The lineskip key needs a value}{%
        For example, use lineskip=2pt or lineskip=-1ex.}%
    \else
      \mathformule@lineskipadjust=#2\relax
    \fi
  \else\ifx\mathformule@key\mathformule@key@lbraceskip
    \ifx\mathformule@value\@empty
      \PackageError{mathformule}{The lbraceskip key needs a value}{%
        For example, use lbraceskip=2pt or lbraceskip=-1pt.}%
    \else
      \mathformule@lbraceadjust=#2\relax
    \fi
  \else\ifx\mathformule@key\mathformule@key@rbraceskip
    \ifx\mathformule@value\@empty
      \PackageError{mathformule}{The rbraceskip key needs a value}{%
        For example, use rbraceskip=2pt or rbraceskip=-1pt.}%
    \else
      \mathformule@rbraceadjust=#2\relax
    \fi
  \else
    \ifx\mathformule@value\@empty
      \mathformule@lineskipadjust=\mathformule@key\relax
    \else
      \PackageError{mathformule}{Unknown formule key `\mathformule@key'}{%
        The supported keys are lineskip, lbrace, rbrace,
        lbraceskip, rbraceskip, and multnum.}%
    \fi
  \fi\fi\fi\fi\fi\fi
}

\def\mathformule@subarabic{\number\mathformule@rowindex}
\def\mathformule@subroman{\romannumeral\mathformule@rowindex}

\def\mathformule@setmultnum#1{%
  \mathformule@multnumtrue
  \def\mathformule@multseparator{.}%
  \let\mathformule@subnumber\mathformule@subarabic
  \def\mathformule@value{#1}%
  \ifx\mathformule@value\@empty
  \else
    \def\mathformule@multstyle{#1}%
    \ifx\mathformule@multstyle\mathformule@style@aa@dot
    \else\ifx\mathformule@multstyle\mathformule@style@aa@dash
      \def\mathformule@multseparator{-}%
    \else\ifx\mathformule@multstyle\mathformule@style@ar@dot
      \let\mathformule@subnumber\mathformule@subroman
    \else\ifx\mathformule@multstyle\mathformule@style@ar@dash
      \def\mathformule@multseparator{-}%
      \let\mathformule@subnumber\mathformule@subroman
    \else\ifx\mathformule@multstyle\mathformule@style@arabic
    \else\ifx\mathformule@multstyle\mathformule@style@roman
      \let\mathformule@subnumber\mathformule@subroman
    \else
      \PackageError{mathformule}{Unknown multnum style
        `\mathformule@multstyle'}{%
        Use arabic.arabic, arabic-arabic, arabic.roman,
        or arabic-roman.}%
    \fi\fi\fi\fi\fi\fi
  \fi
}

\def\mathformule@setbrace#1#2{%
  \ifnum\mathformule@braceside=\z@
    \mathformule@braceside=#1\relax
    \def\mathformule@braceformula{#2}%
  \else
    \ifnum\mathformule@braceside=#1\relax
      \def\mathformule@braceformula{#2}%
    \else
      \PackageError{mathformule}{lbrace and rbrace cannot be combined}{%
        Select only one side for the brace.}%
    \fi
  \fi
}

% multnum uses a full-width alignment whose equation-number cell has zero
% width.  The formula columns therefore remain centred and the tags do not
% become part of the material measured by a surrounding brace.
\newskip\mathformule@leftskip
\newskip\mathformule@rightskip
\newbox\mathformule@multbox
\newbox\mathformule@sidebox
\newdimen\mathformule@formulawidth
\newdimen\mathformule@colwdA
\newdimen\mathformule@colwdB
\newdimen\mathformule@colwdC
\newdimen\mathformule@colwdD
\newdimen\mathformule@colwdE
\newdimen\mathformule@colwdF
\newdimen\mathformule@colwdG
\newdimen\mathformule@colwdH

% Follow the document-wide fleqn convention when \mathindent exists.  In the
% ordinary case equal infinite glues centre the formula independently of its
% zero-width equation tag.
\def\mathformule@preparelayout{%
  \@ifundefined{mathindent}{%
    \mathformule@fleqnfalse
    \mathformule@leftskip=0pt plus 1000pt
    \mathformule@rightskip=0pt plus 1000pt
  }{%
    \mathformule@fleqntrue
    \mathformule@leftskip=\mathindent
    \mathformule@rightskip=0pt plus 1fil
  }%
}

\def\mathformule@resetcolwidths{%
  \global\mathformule@colwdA=\z@
  \global\mathformule@colwdB=\z@
  \global\mathformule@colwdC=\z@
  \global\mathformule@colwdD=\z@
  \global\mathformule@colwdE=\z@
  \global\mathformule@colwdF=\z@
  \global\mathformule@colwdG=\z@
  \global\mathformule@colwdH=\z@
}

\def\mathformule@updatecolwidth#1{%
  \ifcase#1\relax
  \or \ifdim\wd\z@>\mathformule@colwdA
         \global\mathformule@colwdA=\wd\z@ \fi
  \or \ifdim\wd\z@>\mathformule@colwdB
         \global\mathformule@colwdB=\wd\z@ \fi
  \or \ifdim\wd\z@>\mathformule@colwdC
         \global\mathformule@colwdC=\wd\z@ \fi
  \or \ifdim\wd\z@>\mathformule@colwdD
         \global\mathformule@colwdD=\wd\z@ \fi
  \or \ifdim\wd\z@>\mathformule@colwdE
         \global\mathformule@colwdE=\wd\z@ \fi
  \or \ifdim\wd\z@>\mathformule@colwdF
         \global\mathformule@colwdF=\wd\z@ \fi
  \or \ifdim\wd\z@>\mathformule@colwdG
         \global\mathformule@colwdG=\wd\z@ \fi
  \or \ifdim\wd\z@>\mathformule@colwdH
         \global\mathformule@colwdH=\wd\z@ \fi
  \fi
}

% Save and restore reference state that would otherwise be local to the first
% alignment cell.  Labels in every formula column then agree, and optional
% reference packages see a real row counter rather than only the parent.
\def\mathformule@newrowreference{%
  \refstepcounter{mathformulerow}%
  \global\let\mathformule@savedlabel\@currentlabel
  \@ifundefined{@currentHref}{%
    \global\let\mathformule@savedhref\@undefined
  }{%
    \global\let\mathformule@savedhref\@currentHref
  }%
  \@ifundefined{cref@currentlabel}{%
    \global\let\mathformule@savedcreflabel\@undefined
  }{%
    \global\let\mathformule@savedcreflabel\cref@currentlabel
  }%
}

\def\mathformule@restorerowreference{%
  \let\@currentlabel\mathformule@savedlabel
  \def\@currentcounter{mathformulerow}%
  \@ifundefined{mathformule@savedhref}{}{%
    \let\@currentHref\mathformule@savedhref
  }%
  \@ifundefined{mathformule@savedcreflabel}{}{%
    \let\cref@currentlabel\mathformule@savedcreflabel
  }%
}

\def\mathformule@preparemultcell#1{%
  \global\mathformule@cellcount=#1\relax
  \ifnum#1=\@ne
    \global\advance\mathformule@typesetrow\@ne
    \global\advance\mathformule@rowindex\@ne
    \global\mathformule@rowtagtrue
    \mathformule@newrowreference
  \fi
  \mathformule@restorerowreference
}

\def\mathformule@addpairgap{%
  \@tempdima=\wd\z@
  \advance\@tempdima by \ht\z@
  \advance\@tempdima by \dp\z@
  \ifdim\@tempdima=\z@
  \else
    \setbox\tw@=\box\z@
    \setbox\z@\hbox{\qquad\box\tw@}%
  \fi
}

\def\mathformule@rownumber{%
  \mathformule@basenumber
  \mathformule@multseparator
  \mathformule@subnumber
}

\def\mathformule@multtagcell#1{%
  \def\mathformule@tagcontent{#1}%
  \ifx\mathformule@tagcontent\@empty
  \else
    \PackageError{mathformule}{Too many alignment columns with multnum}{%
      multnum supports at most four ampersand-aligned pairs per row.}%
  \fi
  \strut
  \ifmathformule@rowtag
    \hbox to\z@{%
      \hss
      \hbox{%
        \begingroup
          \edef\theequation{\mathformule@rownumber}%
          \@eqnnum
        \endgroup}}%
  \else
    \global\advance\mathformule@rowindex\m@ne
  \fi
}

\protected\def\mathformule@notag{%
  \global\mathformule@rowtagfalse
}

% Complete the fixed eight formula columns.  The conditional must finish
% before the selected macro emits alignment tabs: TeX conditionals may not
% remain open across cells of an alignment.
\def\mathformule@pad@eight{& & & & & & & &}
\def\mathformule@pad@seven{& & & & & & &}
\def\mathformule@pad@six{& & & & & &}
\def\mathformule@pad@five{& & & & &}
\def\mathformule@pad@four{& & & &}
\def\mathformule@pad@three{& & &}
\def\mathformule@pad@two{& &}
\def\mathformule@pad@one{&}
\let\mathformule@pad@none\@empty
\def\mathformule@pad@error{%
  \PackageError{mathformule}{Too many alignment columns with multnum}{%
    multnum supports at most four ampersand-aligned pairs per row.}%
}
\def\mathformule@padmultrow{%
  \ifcase\mathformule@cellcount
    \let\mathformule@next\mathformule@pad@eight
  \or \let\mathformule@next\mathformule@pad@seven
  \or \let\mathformule@next\mathformule@pad@six
  \or \let\mathformule@next\mathformule@pad@five
  \or \let\mathformule@next\mathformule@pad@four
  \or \let\mathformule@next\mathformule@pad@three
  \or \let\mathformule@next\mathformule@pad@two
  \or \let\mathformule@next\mathformule@pad@one
  \or \let\mathformule@next\mathformule@pad@none
  \else \let\mathformule@next\mathformule@pad@error
  \fi
  \mathformule@next
}

% Temporarily balance the current alignment template while the LaTeX kernel
% tests for an optional row adjustment.  This prevents a following alignment
% tab---including one at the start of the next row---from ending the template
% during lookahead.
\protected\def\mathformule@multcr{%
  \relax\iffalse{\fi\ifnum0=`}\fi
  \@ifnextchar[{\mathformule@multcr@i}{\mathformule@multcr@i[0pt]}%
}
\def\mathformule@multcr@i[#1]{%
  \ifnum0=`{\fi\iffalse}\fi
  \let\mathformule@next\mathformule@multcr@normal
  \ifmathformule@trailingbreak
    \ifnum\mathformule@typesetrow=\mathformule@rowcount
      \let\mathformule@next\mathformule@multcr@last
    \fi
  \fi
  \mathformule@next{#1}%
}
\def\mathformule@multcr@normal#1{%
  \mathformule@padmultrow&\cr
  \noalign{%
    \global\mathformule@cellcount=\z@
    \vskip #1\relax}%
}
% A final \\ already closes the last physical row.  Consume the synthetic
% end marker instead of starting an empty, numbered row.  Any adjustment on a
% trailing break has no following row to separate and is therefore ignored.
\def\mathformule@multcr@last#1\mathformule@endmultrow{%
  \mathformule@padmultrow&\crcr
}

\def\mathformule@endmultrow{%
  \mathformule@padmultrow&\crcr
}

\def\mathformule@initoptions{%
  \mathformule@lineskip=\mathformulelineskip
  \mathformule@lineskipadjust=\mathformule@globallineskipadjust
  \mathformule@lbraceadjust=\z@
  \mathformule@rbraceadjust=\z@
  \mathformule@braceside=\z@
  \mathformule@multnumfalse
  \def\mathformule@multseparator{.}%
  \let\mathformule@subnumber\mathformule@subarabic
  \let\mathformule@braceformula\@empty
}

% Count top-level row breaks without expanding the body.  Braced arguments are
% consumed as single items, and an explicit environment depth keeps \\ inside
% array/matrix-like nested environments from activating multnum by accident.
\newcount\mathformule@envdepth
\def\mathformule@scansentinel{\mathformule@scansentinel}
\def\mathformule@sentineltoken{\mathformule@scansentinel}
\def\mathformule@begintoken{\begin}
\def\mathformule@endtoken{\end}
\def\mathformule@breaktoken{\\}
\def\mathformule@ampersandtoken{&}
\long\def\mathformule@countrows#1{%
  \mathformule@rowcount=\@ne
  \mathformule@scancells=\@ne
  \mathformule@envdepth=\z@
  \mathformule@toomanycolsfalse
  \mathformule@trailingbreakfalse
  \mathformule@hasalignmentfalse
  \mathformule@scanrows#1\mathformule@scansentinel
}
\long\def\mathformule@scanrows#1{%
  \def\mathformule@scanitem{#1}%
  \ifx\mathformule@scanitem\mathformule@sentineltoken
    \ifmathformule@trailingbreak
      \advance\mathformule@rowcount\m@ne
    \fi
    \let\mathformule@next\relax
  \else\ifx\mathformule@scanitem\mathformule@begintoken
    \mathformule@trailingbreakfalse
    \let\mathformule@next\mathformule@scanbegin
  \else\ifx\mathformule@scanitem\mathformule@endtoken
    \mathformule@trailingbreakfalse
    \let\mathformule@next\mathformule@scanend
  \else
    \let\mathformule@next\mathformule@scanrows
    \ifnum\mathformule@envdepth=\z@
      \ifx\mathformule@scanitem\mathformule@breaktoken
        \advance\mathformule@rowcount\@ne
        \mathformule@scancells=\@ne
        \mathformule@trailingbreaktrue
        \let\mathformule@next\mathformule@scanbreaktail
      \else\ifx\mathformule@scanitem\mathformule@ampersandtoken
        \mathformule@trailingbreakfalse
        \mathformule@hasalignmenttrue
        \advance\mathformule@scancells\@ne
        \ifnum\mathformule@scancells>8
          \mathformule@toomanycolstrue
        \fi
      \else
        \mathformule@trailingbreakfalse
      \fi\fi
    \fi
  \fi\fi\fi
  \mathformule@next
}
% A row break's optional local spacing belongs to that break, not to the next
% row.  Consume it here so a final \\[<dimension>] is still recognised as a
% trailing break rather than as mathematical content.
\def\mathformule@scanbreaktail{%
  \@ifnextchar[{\mathformule@scanbreakadjust}{\mathformule@scanrows}%
}
\def\mathformule@scanbreakadjust[#1]{\mathformule@scanrows}
\long\def\mathformule@scanbegin#1{%
  \advance\mathformule@envdepth\@ne
  \mathformule@scanrows
}
\long\def\mathformule@scanend#1{%
  \ifnum\mathformule@envdepth>\z@
    \advance\mathformule@envdepth\m@ne
  \fi
  \mathformule@scanrows
}

% A row break accepts an optional *additional* vertical adjustment.  The
% environment-wide separation has already been installed by \openup.
% Use the same alignment-safe optional-argument lookahead in shared-tag mode.
\protected\def\mathformule@cr{%
  \relax\iffalse{\fi\ifnum0=`}\fi
  \@ifnextchar[{\mathformule@cr@i}{\mathformule@cr@i[0pt]}%
}
\def\mathformule@cr@i[#1]{%
  \ifnum0=`{\fi\iffalse}\fi
  \let\mathformule@next\mathformule@cr@normal
  \ifmathformule@trailingbreak
    \ifnum\mathformule@typesetrow=\mathformule@rowcount
      \let\mathformule@next\mathformule@cr@last
    \fi
  \fi
  \mathformule@next{#1}%
}
\def\mathformule@cr@normal#1{%
  \cr
  \noalign{\vskip #1\relax}%
}
\def\mathformule@cr@last#1\mathformule@endsinglerow{\crcr}
\def\mathformule@endsinglerow{\crcr}

% With no top-level ampersand, the sole formula column is left aligned.  Once
% an alignment point is present, retain the conventional right-before/left-
% after column pattern.  The selected fill macros contain no open conditionals
% when TeX crosses an alignment cell boundary.
\let\mathformule@firstbefore\hfil
\let\mathformule@firstafter\@empty
\def\mathformule@preparefirstcolumn{%
  \ifmathformule@hasalignment
    \let\mathformule@firstbefore\hfil
    \let\mathformule@firstafter\@empty
  \else
    \let\mathformule@firstbefore\@empty
    \let\mathformule@firstafter\hfil
  \fi
}

\def\mathformule@beginsharedrow{%
  \global\advance\mathformule@typesetrow\@ne
}

% The full-width outer hbox keeps the tag at the document margin while the
% formula obeys either centred display layout or the class's fleqn indent.
\def\mathformule@tagbox{%
  \hbox to\z@{\hss\hbox{\@eqnnum}}%
}

\long\def\mathformule@singletagtypeset#1{%
    \let\\\mathformule@cr
    % Primitive display math supplies TeX's contextual, rubber vertical
    % spaces: \abovedisplayskip/\abovedisplayshortskip and their below forms.
    \m@th
    \dollardollar@begin
    \hbox to\displaywidth{%
      \hskip\mathformule@leftskip
      $\m@th\displaystyle
        \ifcase\mathformule@braceside
        \or
          \mathformule@braceformula\left\{\kern\mathformule@lbraceadjust
        \or
          \left.
        \fi
        \vcenter{%
          \openup\mathformule@lineskip
          \m@th
          \everycr{}%
          \tabskip\z@skip
          \ialign{%
            \mathformule@beginsharedrow
            \strut\mathformule@firstbefore$\displaystyle{##}$%
            \mathformule@firstafter
            &$\displaystyle{{}##}$\hfil%
            &&\qquad\hfil$\displaystyle{##}$%
            &$\displaystyle{{}##}$\hfil\crcr
            #1\mathformule@endsinglerow
          }%
        }%
        \ifcase\mathformule@braceside
        \or
          \right.
        \or
          \kern\mathformule@rbraceadjust\right\}\mathformule@braceformula
        \fi
      $%
      \hskip\mathformule@rightskip
      \mathformule@tagbox
    }%
    \dollardollar@end
}

\long\def\mathformule@multtypeset#1{%
    \protected@edef\mathformule@basenumber{\theequation}%
    \def\themathformulerow{%
      \mathformule@basenumber
      \mathformule@multseparator
      \mathformule@subnumber}%
    \let\\\mathformule@multcr
    \let\notag\mathformule@notag
    \let\nonumber\mathformule@notag
    \global\mathformule@rowindex=\z@
    \global\mathformule@typesetrow=\z@
    \global\mathformule@cellcount=\z@
    \mathformule@resetcolwidths
    \m@th
    \dollardollar@begin
    \setbox\mathformule@multbox=\vbox{%
      \openup\mathformule@lineskip
      \m@th
      \everycr{}%
      \tabskip\mathformule@leftskip
      \halign to\displaywidth\bgroup
        \mathformule@preparemultcell{1}%
          \setbox\z@\hbox{$\displaystyle{##}$}%
          \mathformule@updatecolwidth{1}%
          \mathformule@firstbefore\box\z@\mathformule@firstafter
          \tabskip\z@skip
        &\mathformule@preparemultcell{2}%
          \setbox\z@\hbox{$\displaystyle{{}##}$}%
          \mathformule@updatecolwidth{2}\box\z@\hfil
          \tabskip\z@skip
        &\mathformule@preparemultcell{3}%
          \setbox\z@\hbox{$\displaystyle{##}$}%
          \mathformule@addpairgap
          \mathformule@updatecolwidth{3}\hfil\box\z@
          \tabskip\z@skip
        &\mathformule@preparemultcell{4}%
          \setbox\z@\hbox{$\displaystyle{{}##}$}%
          \mathformule@updatecolwidth{4}\box\z@\hfil
          \tabskip\z@skip
        &\mathformule@preparemultcell{5}%
          \setbox\z@\hbox{$\displaystyle{##}$}%
          \mathformule@addpairgap
          \mathformule@updatecolwidth{5}\hfil\box\z@
          \tabskip\z@skip
        &\mathformule@preparemultcell{6}%
          \setbox\z@\hbox{$\displaystyle{{}##}$}%
          \mathformule@updatecolwidth{6}\box\z@\hfil
          \tabskip\z@skip
        &\mathformule@preparemultcell{7}%
          \setbox\z@\hbox{$\displaystyle{##}$}%
          \mathformule@addpairgap
          \mathformule@updatecolwidth{7}\hfil\box\z@
          \tabskip\z@skip
        &\mathformule@preparemultcell{8}%
          \setbox\z@\hbox{$\displaystyle{{}##}$}%
          \mathformule@updatecolwidth{8}\box\z@\hfil
          \tabskip\mathformule@rightskip
        &\mathformule@multtagcell{##}%
          \tabskip\z@skip\crcr
        #1\mathformule@endmultrow
      \egroup
    }%
    \mathformule@formulawidth=\mathformule@colwdA
    \advance\mathformule@formulawidth by \mathformule@colwdB
    \advance\mathformule@formulawidth by \mathformule@colwdC
    \advance\mathformule@formulawidth by \mathformule@colwdD
    \advance\mathformule@formulawidth by \mathformule@colwdE
    \advance\mathformule@formulawidth by \mathformule@colwdF
    \advance\mathformule@formulawidth by \mathformule@colwdG
    \advance\mathformule@formulawidth by \mathformule@colwdH
    \ifmathformule@fleqn
      \@tempdima=\mathformule@leftskip
    \else
      \@tempdima=\displaywidth
      \advance\@tempdima by -\mathformule@formulawidth
      \divide\@tempdima\tw@
    \fi
    \ifcase\mathformule@braceside
    \or
      \setbox\mathformule@sidebox=\hbox{$\displaystyle
        \mathformule@braceformula
        \left\{\vphantom{\vcenter{\copy\mathformule@multbox}}\right.$}%
      \hbox to\z@{%
        \kern\@tempdima
        \kern-\mathformule@lbraceadjust
        \kern-\wd\mathformule@sidebox
        \box\mathformule@sidebox\hss}%
    \or
      \setbox\mathformule@sidebox=\hbox{$\displaystyle
        \left.\vphantom{\vcenter{\copy\mathformule@multbox}}\right\}%
        \mathformule@braceformula$}%
      \hbox to\z@{%
        \kern\@tempdima
        \kern\mathformule@formulawidth
        \kern\mathformule@rbraceadjust
        \box\mathformule@sidebox\hss}%
    \fi
    \vcenter{\box\mathformule@multbox}%
    \dollardollar@end
}

\long\def\mathformule@dispatch#1#2{%
  \ifmmode
    \PackageError{mathformule}{formule cannot be nested in math mode}{%
      End the surrounding math mode before starting formule.}%
  \else
    \begingroup
      \mathformule@initoptions
      \mathformule@parseoptions{#1}%
      \advance\mathformule@lineskip by \mathformule@lineskipadjust
      \mathformule@preparelayout
      \mathformule@countrows{#2}%
      \mathformule@preparefirstcolumn
      \global\mathformule@typesetrow=\z@
      \refstepcounter{equation}%
      \ifmathformule@multnum
        \ifnum\mathformule@rowcount>\@ne
          \ifmathformule@toomanycols
            \PackageError{mathformule}{Too many alignment columns with multnum}{%
              multnum supports at most four ampersand-aligned pairs per row.}%
            \mathformule@singletagtypeset{#2}%
          \else
            \mathformule@multtypeset{#2}%
          \fi
        \else
          \mathformule@singletagtypeset{#2}%
        \fi
      \else
        \mathformule@singletagtypeset{#2}%
      \fi
    \endgroup
    \@ignoretrue
  \fi
}

\def\mathformule@targetenv{formule}
\protected\long\def\mathformule@collect#1{%
  \mathformule@collect@i{#1}{}%
}
\long\def\mathformule@collect@i#1#2#3\end#4{%
  \def\mathformule@foundenv{#4}%
  \ifx\mathformule@foundenv\mathformule@targetenv
    \let\mathformule@next\mathformule@collect@finish
  \else
    \let\mathformule@next\mathformule@collect@continue
  \fi
  \mathformule@next{#1}{#2#3}{#4}%
}
\long\def\mathformule@collect@finish#1#2#3{%
  \mathformule@dispatch{#1}{#2}%
  \end{#3}%
}
\long\def\mathformule@collect@continue#1#2#3{%
  \mathformule@collect@i{#1}{#2\end{#3}}%
}

% The optional argument accepts keys and retains the old [<dimension>] form.
% Capturing the body lets multnum distinguish a genuine multiline display
% without executing or typesetting the formula twice.  Refuse to replace an
% environment (or either of its defining commands) supplied by another file.
\def\mathformule@nameclash{%
  \PackageError{mathformule}{Environment `formule' is already defined}{%
    Rename or remove the conflicting definition before loading mathformule.}%
}
\newif\ifmathformule@environmentdefined
\@ifundefined{formule}{%
  \@ifundefined{endformule}{%
    \newenvironment{formule}[1][]
      {\mathformule@collect{#1}}
      {}%
    \mathformule@environmentdefinedtrue
  }{\mathformule@nameclash}%
}{\mathformule@nameclash}

% The experimental tagged-math layer in current LaTeX grabs complete
% environments before they start.  Our ordinary definition already performs
% that job itself, so in this one configuration use the kernel's built-in body
% argument and notify its math processor explicitly.  The branch is dormant
% on conventional LaTeX runs and does not load an auxiliary package.
\def\mathformule@enabletaggedmath{%
  \RenewDocumentEnvironment{formule}{O{} +b}{%
    \csname __math_process:nn\endcsname{formule}{##2}%
    \@kernel@math@registered@begin
    \mathformule@dispatch{##1}{##2}%
  }{}%
}
\AtBeginDocument{%
  \ifmathformule@environmentdefined
    \@ifundefined{@kernel@math@registered@begin}{}{%
      \mathformule@enabletaggedmath
    }%
  \fi
}

\makeatother

\endinput