split

<..equations of amsmath.sty..>+
 \NewConfigure{split}{6}
                                     \catcode‘\#=13 \catcode‘\!=6
 \def\:tempc{%
    \vbox\bgroup
         <.insplit 1999.>%
         \Let@  \restore@math@cr  \default@tag \everycr{}\a:split
         \SaveMkHalignConf:g{split}\HRestore\noalign
         \MkHalign#{<.amsmath split pattern.>}}
 \HLet\insplit@=\:tempc
                                     \catcode‘\#=6 \catcode‘\!=12
 \def\:tempc{%
    \crcr\EndMkHalign \b:split \egroup }
 \HLet\endsplit=\:tempc
 -_-_-

<..insplit 1999..>
 \ifx \ifinany@\:Undef
    \chardef\dspbrk@context\@ne
 \fi
 -_-_-

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

<..amsmath 1999..>
 \let\:tempc=\gather@split
 \append:defIII\:tempc{%
    \def\endmathdisplay@a{%
      \math@cr \EndMkHalign
      \RecallMkHalignConfig \csname b:gather\ifst@rred *\fi\endcsname
      \egroup on \vcenter\bgroup
    }%
 }
 \HLet\gather@split=\:tempc
 -_-_-

The split environment looks like ‘\begin{split}...\begin{split}...\end{split}...\endmathdisplay@a’. The folowing tries to compensate for the above extra openning split environment.

<..equations of amsmath.sty..>+
 \Configure{@begin}
   {split}
   {\relax
    \ifx  \split\insplit@ \else
    \def\choose:begin{\global\let\choose:begin\@firstoftwo
                      \@secondoftwo}%
    \fi
   }
 -_-_-

The split environment still has problems