input: Write a python function to add two 3x3 matrices.
  assert add_matrices([[1, 2, 3], [4, 5, 6], [7, 8, 9]], [[10, 11, 12], [13, 14, 15], [16, 17, 18]]) == [[11, 13, 15], [17, 19, 21], [23, 25, 27]]
  assert add_matrices([[0, 0, 0], [0, 0, 0], [0, 0, 0]], [[1, 1, 1], [1, 1, 1], [1, 1, 1]]) == [[1, 1, 1], [1, 1, 1], [1, 1, 1]]
  assert add_matrices([[1, 1, 1], [1, 1, 1], [1, 1, 1]], [[-1, -1, -1], [-1, -1, -1], [-1, -1, -1]]) == [[0, 0, 0], [0, 0, 0], [0, 0, 0]]
output: def add_matrices(mat1, mat2):
      return [[mat1[i][j] + mat2[i][j] for j in range(3)] for i in range(3)]