input: 
  def add_matrix(matrix1: List[List[int]], matrix2: List[List[int]]) -> Union[str, List[List[int]]]:
      """
      Add two matrices if they are compatible (i.e., they have the same dimensions).
      Otherwise, return an error message.
      
      >>> add_matrix([[1, 2], [3, 4]], [[5, 6], [7, 8]])
      [[6, 8], [10, 12]]
      
      >>> add_matrix([[1, 2], [3, 4]], [[5, 6, 7], [8, 9, 10]])
      'Matrices not compatible'
      """
      if len(matrix1) != len(matrix2) or len(matrix1[0]) != len(matrix2[0]):
          return 'Matrices not compatible'
      
      result = [[matrix1[i][j] + matrix2[i][j] for j in range(len(matrix1[0]))] for i in range(len(matrix1))]
      return result
  
output:     if len(matrix1) != len(matrix2) or len(matrix1[0]) != len(matrix2[0]):
          return 'Matrices not compatible'
      
      result = [[matrix1[i][j] + matrix2[i][j] for j in range(len(matrix1[0]))] for i in range(len(matrix1))]
      return result