The last four parameters of pictorial configurations must be empty.
<..plain,latex math..>+
\def\:tempc#1{\tx:halign{matrix}{#1}}
\HLet\matrix=\:tempc
\catcode‘\#13 \catcode‘\!6
\def\reg:matrix!1{\null\,\vcenter{\normalbaselines\m@th
\SaveMkHalignConf:g{matrix}\RecallTeXcr
\MkHalign#{$#$&&$#$}!1\crcr
\EndMkHalign\RecallMkHalignConfig}}
\catcode‘\#=6 \catcode‘\!=12
\NewConfigure{matrix}{6}
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