\begin{calstable}
\colwidths{{100pt}{100pt}}

\brow
\nullcell{ltb}\nullcell{rtb}\spancontent{row1}
\show\cals@ifInRspan
\cals@ifLastRspanRow\iftrue \show L \else \show X \fi
\erow

\brow
\nullcell{ltr}\cell{r2}
\show\cals@ifInRspan
\cals@ifLastRspanRow\iftrue \show L \else \show X \fi
\erow

\brow
\nullcell{lbr}\spancontent{r3}\nullcell{ltr}
\show\cals@ifInRspan
\cals@ifLastRspanRow\iftrue \show L \else \show X \fi
\erow

\brow
\cell{r3}\nullcell{lbr}\spancontent{r4}
\show\cals@ifInRspan
\cals@ifLastRspanRow\iftrue \show L \else \show X \fi
\erow

\brow
\cell{r4c1}
\cell{r4c2}
\show\cals@ifInRspan
\cals@ifLastRspanRow\iftrue \show L \else \show X \fi
\erow

\end{calstable}