input: Write a function to add two matrices provided as input. If the matrices are not compatible for addition due to differing dimensions, return a message indicating so. Otherwise, return the resulting matrix obtained by adding the corresponding elements 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]]) == [[4, 6]]
  assert add_matrix([[1, 2], [3, 4]], [[5], [6]]) == 'Matrices not compatible'
  assert add_matrix([[1], [2]], [[3, 4], [5, 6]]) == 'Matrices not compatible'
  assert add_matrix([], []) == []
output: def add_matrix(matrix1, matrix2):
      if len(matrix1) != len(matrix2) or any(len(row1) != len(row2) for row1, row2 in zip(matrix1, matrix2)):
          return 'Matrices not compatible'
      return [[a + b for a, b in zip(row1, row2)] for row1, row2 in zip(matrix1, matrix2)]