TeX: Eqalign

<..plain eqalign pattern..>
 $\displaystyle{#}$&$\displaystyle{{}#}$%
 -_-_-

<..plain halign-based math..>
 \def\:tempc#1{\tx:halign{eqalign}{#1}}
 \HLet\eqalign=\:tempc
                                  \catcode‘\#=13 \catcode‘\!=6
 \def\reg:eqalign!1{\null\vcenter{\m@th
   \Mk:ialign:egroup
   {<.plain eqalign pattern.>}{eqalign}{!1}#{}}}
                                  \catcode‘\#=6 \catcode‘\!=12
 \NewConfigure{eqalign}{6}
 -_-_-

<..plain halign-based math..>+
 \def\Mk:ialign:egroup#1#2#3#4#5{%
 %    \csname a:#2\endcsname
      \ifx \EndPicture\:UnDef
         <.SaveMkHalignConfig.>#5\RecallTeXcr
         \MkHalign#4{#1}#3\crcr\EndMkHalign
         \RecallMkHalignConfig
      \else
         \csname o:#2:\endcsname{#3}%
      \fi
 %   \csname b:#2\endcsname
 }
 -_-_-

Test: