mat.dirsum: Forms the direct sum of a list of matrices
Description
The direct sum is the partitioned matrices whose diagonal submatrices are
the matrices from which the direct sum is to be formed and whose off-diagonal
submatrices are conformable matrices of zeroes. The resulting
matrix is \(m \times n\), where \(m\) is the sum of
the numbers of rows of the contributing matrices and \(n\) is the sum of
their numbers of columns.