\section{SUBSINDEX Overloaded Class Indexing}

\subsection{Usage}

This method is called for classes in the expressions
of the form
\begin{verbatim}
  c = subsindex(a)
\end{verbatim}
where \verb|a| is an object, and \verb|c| is an index vector.
It is also called for
\begin{verbatim}
  c = b(a)
\end{verbatim}
in which case \verb|subsindex(a)| must return a vector containing
integers between \verb|0| and \verb|N-1| where \verb|N| is the number
of elements in the vector \verb|b|.
