__remove_whitespaces(s) removed. test_P12  changed to:

def test_P12():
    A11 = MatrixSymbol('A11',n,n)
    A12 = MatrixSymbol('A12',n,n)
    A22 = MatrixSymbol('A22',n,n)
    B = BlockMatrix([[A11,A12],[ZeroMatrix(n,n),A22]])
    assert block_collapse(B.I)  == BlockMatrix([
                                    [A11.I,          (-1)*A11.I*A12*A22.I],
                                    [ZeroMatrix(n,n), A22.I]])