input: Write a python function to multiply two 2x2 matrices.
  assert matrix_mul([[1, 2], [3, 4]], [[5, 6], [7, 8]]) == [[19, 22], [43, 50]]
  assert matrix_mul([[2, 0], [1, 2]], [[1, 3], [4, 0]]) == [[2, 6], [9, 3]]
  assert matrix_mul([[0, 0], [0, 0]], [[1, 1], [1, 1]]) == [[0, 0], [0, 0]]
output: def matrix_mul(A, B):
      return [
          [
              A[0][0] * B[0][0] + A[0][1] * B[1][0],
              A[0][0] * B[0][1] + A[0][1] * B[1][1]
          ],
          [
              A[1][0] * B[0][0] + A[1][1] * B[1][0],
              A[1][0] * B[0][1] + A[1][1] * B[1][1]
          ]
      ]