amsmath-multline.tex

\begin{multline*}
f = \begin{matrix} x \end{matrix}
\end{multline*}


\newenvironment{multline*}{\start@multline\st@rredtrue}{\endmultline}

% expand \multline*
\start@multline\st@rredtrue
f = \begin{matrix} x \end{matrix}
\end{multline*}


\def\start@multline#1{%
    \RIfM@ % \def\RIfM@{\relax\ifmmode}
        \nomath@env
        \DN@{\@namedef{end\@currenvir}{}\@gobble}% \def\DN@{\def\next@}
    \else
        $$%
        #1%
        \ifst@rred
            \nonumber
        \else
            \global\@eqnswtrue
        \fi
        \let\next@\multline@
    \fi
    \collect@body\next@
}
$$

% expand \start@multline, #1 = "\st@rredtrue"
% expand \RIfM@, false
\double@dollar%
\st@rredtrue
\ifst@rred
    \nonumber
\else
    \global\@eqnswtrue
\fi
\let\next@\multline@
\collect@body\next@
f = \begin{matrix} x \end{matrix}
\end{multline*}


% expand \start@multline, #1 = "\st@rredtrue"
% expand \RIfM@, false
% expand \st@rredtrue, skip until \nonumber
% \next@ -> let to \multline@
\double@dollar
\collect@body\next@
f = \begin{matrix} x \end{matrix}
\end{multline*}


\def\collect@body#1{%
  \@envbody{\@xp#1\@xp{\the\@envbody}}%
  \edef\process@envbody{\the\@envbody\@nx\end{\@currenvir}}%
  \@envbody\@emptytoks \def\begin@stack{b}%
  \begingroup
  \@xp\let\csname\@currenvir\endcsname\collect@@body
  \edef\process@envbody{\@xp\@nx\csname\@currenvir\endcsname}%
  \process@envbody % equivalent to \collect@@body
}

% expand \collect@body, #1 = "\next@"
% \next@ -> let to \multline@
% \@envbody -> toks "\@xp\next@\@xp{\the\@envbody}"
% \process@envbody -> "\@xp\next@\@xp{\the\@envbody}\end{multline*}"
% \@envbody -> toks ""
% \begin@stack -> "b"
% (start new group)
% (local) \multline* -> let to \collect@@body
% (local) \process@envbody -> "\multline*"
\double@dollar
\begingroup
\process@envbody % equivalent to \collect@@body
f = \begin{matrix} x \end{matrix}
\end{multline*}


\def\collect@@body#1\end#2{%
  \edef\begin@stack{\push@begins#1\begin\end \@xp\@gobble\begin@stack}%
  \ifx\@empty\begin@stack
    \endgroup
    \@checkend{#2}%
    \addto@envbody{#1}%
  \else
    \addto@envbody{#1\end{#2}}%
  \fi
  \process@envbody % A little tricky! Note the grouping
}

% expand \process@envbody, #1 = "f = \begin{matrix} x ", #2 = "matrix"
%  - if #1 contains \begin{...}, \begin@stack is nonempty, continue collecting, 
%     (local) \process@envbody is equal to \collect@@body
%  - otherwise, \begin@stack is empty, end the group, end the collection, 
%     \process@envbody -> "\@xp\next@\@xp{\the\@envbody}\end{multline*}"
\double@dollar
\begingroup
\edef\begin@stack{\push@begins f = \begin{matrix} x \begin\end \@xp\@gobble\begin@stack}%
\ifx\@empty\begin@stack
  \endgroup
  \@checkend{matrix}%
  \addto@envbody{f = \begin{matrix} x }%
\else
  \addto@envbody{f = \begin{matrix} x \end{matrix}}%
\fi
\process@envbody % A little tricky! Note the grouping
\end{multline*}