input: Write a function to add two matrices. If the matrices are not of the same size, return 'Matrices not compatible'. Otherwise, return the sum of the two matrices.
  assert add_matrix([[1, 2], [3, 4]], [[5, 6], [7, 8]])==[[6, 8], [10, 12]]
  assert add_matrix([[9, 8], [7, 6]], [[5, 4], [3, 2]])==[[14, 12], [10, 8]]
  assert add_matrix([[1, 1], [1, 1]], [[2, 2], [2, 2]])==[[3, 3], [3, 3]]
  assert add_matrix([[1]], [[2]])==[[3]]
  assert add_matrix([[1, 2, 3]], [[4, 5, 6]])==[[5, 7, 9]]
output: def add_matrix(mat1, mat2):
      if len(mat1) != len(mat2) or any(len(row1) != len(row2) for row1, row2 in zip(mat1, mat2)):
          return 'Matrices not compatible'
      return [[mat1[i][j] + mat2[i][j] for j in range(len(mat1[0]))] for i in range(len(mat1))]