\begin{calstable}
\colwidths{{100pt}}
\brow
\def\cals@AtBeginCell{\show A}
\def\cals@AtEndCell{\show B}
\cell{a}
\def\cals@AtBeginCell{\show C}
\def\cals@AtEndCell{\show D}
\nullcell{ltb}
\nullcell{rtb}\spancontent{b}
\erow
\end{calstable}
