code: def block_matrix(A, B, C, D):
      r
      return vstackm((hstackm((A, B)), hstackm((C, D))))
docs: r"""Generate the operator matrix with quadrants
  
      .. math::
  
         \begin{pmatrix} A B \\ C D \end{pmatrix}
  
      Args:
          A (Matrix): Matrix of shape ``(n, m)``
          B (Matrix): Matrix of shape ``(n, k)``
          C (Matrix): Matrix of shape ``(l, m)``
          D (Matrix): Matrix of shape ``(l, k)``
  
      Returns:
          Matrix: The combined block matrix ``[[A, B], [C, D]]``.