% Allow table in colspan
% No warnings are expected:
%  You can't use `\\prevdepth' in horizontal mode.
%  Missing $ inserted.
%  etc.

\par
\begin{calstable}
\colwidths{{1in}{1in}{1in}{1in}}
\brow
\cell{normal}
\tracingall

\nullcell{ltb}
\nullcell{rtb}
\spancontent{
  xxx\par

  \begin{calstable}
  \colwidths{{1in}}
  \brow\cell{inner table}\erow
  \end{calstable}
}
\par

\cell{normal}
%\tracingall
%\par
\erow
\end{calstable}

\end{document}