input: Write a function to add two matrices. If the matrices are not compatible for addition due to different dimensions, return 'Matrices not compatible'. Otherwise, return the resultant matrix after addition.
  assert add_matrix(
      [[1, 2], 
       [3, 4]], 
      [[5, 6], 
       [7, 8]]
  ) == [[6, 8], 
        [10, 12]]
  assert add_matrix(
      [[1, 2]], 
      [[5, 6]]
  ) == [[6, 8]]
  assert add_matrix(
      [[1, 2], 
       [3, 4]], 
      [[5, 6]]
  ) == 'Matrices not compatible'
  assert add_matrix(
      [[1, 2, 3], 
       [4, 5, 6]], 
      [[10, 11, 12], 
       [13, 14, 15]]
  ) == [[11, 13, 15], 
        [17, 19, 21]]
  assert add_matrix([], []) == 'Matrices not compatible'
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 [[matrix1[i][j] + matrix2[i][j] for j in range(len(matrix1[0]))] for i in range(len(matrix1))]