\begin{Schunk}
\begin{Sinput}
 dim(x1a)
\end{Sinput}
\begin{Soutput}
NULL
\end{Soutput}
\begin{Sinput}
 dim(X)
\end{Sinput}
\begin{Soutput}
[1] 4 6
\end{Soutput}
\end{Schunk}
