in latex.ltx

\def\equation{$$\refstepcounter{equation}}
\def\endequation{\eqno \hbox{\@eqnnum}$$\global\@ignoretrue}

<..latex.ltx ..>
\let\o:equation:=\equation
\let\o:endequation:=\endequation
\def\equation{\bgroup
    \ifx \EndPicture\:Undef
       \def\endequation{%
           \if@eqnsw \expand:after{\o:endequation:\b:equation\equ:no}%
           \else \expand:after{\o:endequation:\egroup}\fi
           \c:equation\egroup}%
       \expandafter\a:equation
    \else
          \def\endequation{\o:endequation:\egroup}
    \fi
    \o:equation:}
-_-_-

<..config latex.ltx shared ..>
\NewConfigure{equation}[3]{%
\c:def\a:equation{#1\bgroup\let\@eqnnum=\empty}%
\c:def\b:equation{\egroup#2}\c:def\c:equation{#3}}
\def\equ:no{\@eqnnum}
\Configure{equation}{}{}{}
-_-_-

<..equations of amsmath.sty ..>
\def\equation{\bgroup <.amsmath-fleqn false .>%
   \ifx \EndPicture\:Undef
      \def\endequation{%
        \let\tagform@=\:gobble
        \if@eqnsw \expand:after{%
           <.remove amsmath-fleqn number .>%
           \o:endequation:\b:equation\equ:no}%
        \else \expand:after{%
           \ifx\df@tag\@empty \expand:after{\o:endequation:\egroup}%
           \else
              \global\let\df:tag=\df@tag
              \expand:after{\o:endequation: \expandafter\b:equation\df:tag
              }\fi
         }\fi
         \c:equation\egroup
       }%
       \expandafter\a:equation
    \else
       \def\endequation{\o:endequation:\egroup}%
    \fi
    \o:equation:}
-_-_-

<..equations of amsmath.sty ..>+
\expandafter\let\csname o:equation*:\expandafter
        \endcsname \csname equation*\endcsname
\expandafter\let\csname o:endequation*:\expandafter
        \endcsname \csname endequation*\endcsname
\expandafter\def\csname equation*\endcsname{\bgroup
   \ifx \EndPicture\:Undef
      \expandafter\def\csname endequation*\endcsname {%
         \csname o:endequation*:\endcsname\egroup%
         \csname b:equation*\endcsname\egroup
       }%
       \csname a:equation*\endcsname
    \else
       \expandafter\def\csname endequation*\endcsname{\csname
                                  o:endequation*:\endcsname\egroup}%
    \fi
    \csname o:equation*:\endcsname}
\NewConfigure{equation*}[2]{
    \expandafter\c:def \csname a:equation*\endcsname {#1\bgroup \let \@eqnnum \empty }%
    \expandafter\c:def \csname b:equation*\endcsname {#2}%
}
-_-_-

<..amsmath-fleqn false ..>
\@fleqnfalse
-_-_-

<..remove amsmath-fleqn numberNO ..>
\if@fleqn
   \pend:def\endmathdisplay@fleqn{\global\let\df:tag=\df@tag
                        \gdef\df@tag{\global\let\df@tag=\df:tag}}%
\fi
-_-_-
\NewConfigure{equation}[3]{%
   \c:def\a:equation{#1\bgroup \let\tagform@=\:gobble }%
   \c:def\b:equation{\egroup#2}\c:def\c:equation{#3}} -_-_-

Do we need the ‘\Configure{Picture*}{}{}%’?

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