如：
A:=Matrix([[1,2],[2,3]]);B:=Matrix([[1,2,3],[1,2,3]]);
怎样得到
C := Matrix(4, 3, {(1, 1) = 1, (1, 2) = 2, (1, 3) = 0, (2, 1) = 2, (2, 2) = 3, (2, 3) = 0, (3, 1) = 1, (3, 2) = 2, (3, 3) = 3, (4, 1) = 1, (4, 2) = 2, (4, 3) = 3})

另外，怎么中以取一个矩阵中的某个子矩阵？
如A：＝Matrix([[1,2,3],[2,3,4]])怎么可以取到A的前两列？或者A的前两行前两列组成的子矩阵？