x33 = [0, 0, 0; 0, 0, 0; 0, 0, 0];
x23 = [0, 0, 0; 0, 0, 0];
(zeros (3) == x33 && zeros (size (x33)) == x33 && zeros (3, 3) == x33
 && zeros (2, 3) == x23 && zeros (3, 2) == x23')
