Braced Tables

Extract the cells recursively.

<..HTable..>+
 \long\def\:HTable#1&#2\EndH:Table{%
    \def\:htbend{#2}%
    \insert:TD#1\tok:nmath
    \def\:temp{#2}\ifx\:temp\empty \EndHTable
    \else  \expand:after{\end:cell
                         \:HTable#2\EndH:Table}\fi }
 \def\tok:nmath{\empty}
 \def\end:cell{\:EndHTableScript   \f:HTable}
 -_-_-

<..HTable..>+
 \def\:cr{\ifx \EndPicture\:UnDef \expandafter\:cR
           \else                  \expandafter\:c:r\fi}
 \expandafter\let\csname :c:r\endcsname=\cr
 \long\def\:cR#1{\relax
    \ifx\:htbend\empty
       \def\:temp{#1}%
       \ifx\:temp\NoArg \else \expandafter\mpty:cr\fi
    \else                      \mpty:cr
    \fi        #1}%
 
 \def\mpty:cr{%
    \expand:after{\end:cell  \gHAdvance\HRow by 1  \gHAssign\HCol=0
    \d:HTable\c:HTable \insert:TD}}
 -_-_-

We need global increases in Col and Row for cases like ‘\HTableScript{$}{$}’.