x([[X, Z], [ZeroMatrix(m, n), Y]])
    >>> print(B)
    Matrix([
    [X, Z],
    [0, Y]])

    >>> C = BlockMatrix([[Identity(n), Z]])
    >>> print(C)
    Matrix([[I, Z]])

    >>> print(block_collapse(C*B))
    Matrix([[X, Z + Z*Y]])
    r