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