 array, which is a
one-dimensional array of matrices. More than one index may be used,
so that <code>m[1, 2]</code> is of type <code>matrix</code> and denotes the matrix
in the first row and second column of the array. Continuing to add
indices, <code>m[1, 2, 3]</code> is of type <code>row_vector</code> and denotes
the third row of the matrix denoted by <code>m[1, 2]</code>. Finally,
<code>m[1, 2, 3, 4]</code> is of type <code>real</code> and denotes the value in the
third row and fourth column of the matri