TeX: eqalignno

<..plain eqalignno pattern..>
 $\@lign\displaystyle{#}$%
 &$\@lign\displaystyle{{}#}$%
 &\hbox{$\@lign#$}%
 -_-_-

<..plain halign-based math..>+
 \def\:tempc#1{\tx:halign{eqalignno}{#1}}
 \HLet\eqalignno=\:tempc
                                  \catcode‘\#=13 \catcode‘\!=6
 \def\reg:eqalignno!1{\Mk:ialign:egroup
    {<.plain eqalignno pattern.>}{eqalignno}{!1}#{}}
                                  \catcode‘\#=6 \catcode‘\!=12
 \NewConfigure{eqalignno}{6}
 \def\:eqalignno:{\Configure{noalign}{}{}}
 -_-_-