input: 
  from typing import List, Union
  
  def add_matrix(matrix1: List[List[float]], matrix2: List[List[float]]) -> Union[List[List[float]], str]:
      """
      Adds two matrices of the same dimensions. If the matrices are not of the same dimensions,
      returns 'Matrices not compatible'.
      
      >>> add_matrix([[1, 2], [3, 4]], [[5, 6], [7, 8]])
      [[6, 8], [10, 12]]
      >>> add_matrix([[1, 2]], [[5, 6], [7, 8]])
      'Matrices not compatible'
      """
      if len(matrix1) != len(matrix2):
          return 'Matrices not compatible'
      if 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):
          return 'Matrices not compatible'
      if 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