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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def block_matrix(A, B, C, D):
 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]]``. """

    return vstackm((hstackm((A, B)), hstackm((C, D))))