gather

<..equations of amsmath.sty..>+
                                     \catcode‘\#=13 \catcode‘\!=6
 \def\reg:gather!1{%
    \ingather@true  <.gather 1999.>%
    \let\tag\tag@in@align  \let\label\label@in@display
    \intertext@ \displ@y@
    \Let@  \let\math@cr@@@\math@cr@@@gather  \gmeasure@{#1}%
    \global\shifttag@false    \global\row@\@ne
   <.halign amsmath gather.>}
                                     \catcode‘\#=6 \catcode‘\!=12
 -_-_-

<..gather 1999..>
 \ifx \ifinany@\:Undef
    \let\split\insplit@   \chardef\dspbrk@context\z@
 \else
    \inany@true
 \fi
 -_-_-

<..halign amsmath gather..>
 \SaveMkHalignConf:g{gather\ifst@rred *\fi}\HRestore\noalign
 \MkHalign#{<.amsmath gather pattern.>}!1%
 -_-_-

<..amsmath gather pattern..>
 \hbox{$\m@th\displaystyle{<.sub/sup base.>#}$}%
 &\hbox{<.sub/sup base.>#}%
 -_-_-

<..equations of amsmath.sty..>+
 \def\:tempc{\math@cr \EndMkHalign
    \RecallMkHalignConfig \csname b:gather\ifst@rred *\fi\endcsname
    $$\global\@ignoretrue  }
 \HLet\endgather=\:tempc
 \def\:tempc{\pic:MkHalign{gather\ifst@rred *\fi}}
 \HLet\gather@=\:tempc
 \NewConfigure{gather}{6}
 \@xp\let\csname reg:gather*\endcsname=\reg:gather
 \@xp\let\csname endgather*\endcsname=\endgather
 \NewConfigure{gather*}{6}
 -_-_-

<..equations of amsmath.sty..>+
 \def\:tempc{%
   \def\intertext##1{%
     \ifvmode\else\\\@empty\fi
     \noalign{%
       \penalty\postdisplaypenalty\vskip\belowdisplayskip
       \vbox{\normalbaselines
         \ifdim\linewidth=\columnwidth
         \else \parshape\@ne \@totalleftmargin \linewidth
         \fi
        \a:intertext \noindent##1\b:intertext\par}%
       \penalty\predisplaypenalty\vskip\abovedisplayskip%
     }%
 }}
 \HLet\intertext@=\:tempc
 \NewConfigure{intertext}{2}
 -_-_-