             If an error occurs on the call, `n' is undefined.
 
   vrtces         is a contiguous set of vertices. The returned
                  vertices are arranged in order of increasing vertex
                  ID. The IDs of the returned vertices range from

                     start

                  to

                     start + n - 1

                  Each vertex is a 3-vector. The correspondence of
                  elements of `vrtces' with the elements of the set of
                  vert