Removed ‘\edef\:temp{\the\everymath}\ifx \:temp\empty \:warning{tabular within math?}\fi
\let\:@tabular:|=\empty’ to allow dealing with
\documentclass{article}%
\begin{document}
\catcode‘\:=11
\def\fff#1{\immediate
\write16{....[#1]..........\the\everymath}%
% \write16{....[#1]..........\meaning\a:mth}%
% \immediate \write16{.............\meaning\PicMath }%
}
\ifx \Configure\UnDef\else
\Configure{$}{\PicMath}{\EndPicMath}{}
\fi
\fff{}
$A$
\begin{tabular}{l}
$C$
\begin{tabular}{l}%
$D$
\end{tabular}
\end{tabular}
$
\begin{array}{l}
$C$
\begin{tabular}{l}%
$D$
\end{tabular}
\end{array}
$
\end{document}