input: Write a function to add two matrices. If the matrices are not compatible (i.e., they do not have the same dimensions), 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([[1, 2, 3], [4, 5, 6]], [[7, 8, 9], [10, 11, 12]])==[[8, 10, 12], [14, 16, 18]]
  assert add_matrix([[1, 2], [3, 4]], [[5, 6]])=='Matrices not compatible'
  assert add_matrix([[1, 2], [3, 4]], [[5, 6], [7, 8], [9, 10]])=='Matrices not compatible'
  assert add_matrix([], [])=='Matrices not compatible'
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))]