input: def add_matrix(matrix1, matrix2):
      """Add two matrices together if they have the same dimensions.
      If the matrices are not compatible (i.e., they do not have the same number of rows and columns),
      return 'Matrices not compatible'.
      
      >>> add_matrix([[1, 2], [3, 4]], [[5, 6], [7, 8]])
      [[6, 8], [10, 12]]
      
      >>> add_matrix([[1, 2], [3, 4]], [[1, 2, 3], [4, 5, 6]])
      'Matrices not compatible'
      
      >>> add_matrix([[1, 2], [3, 4]], [[5, 6], [7, 8]])
      [[6, 8], [10, 12]]
      """
      if len(matrix1) != len(matrix2) or len(matrix1[0]) != len(matrix2[0]):
          return 'Matrices not compatible'
      
      return [[matrix1[i][j] + matrix2[i][j] for j in range(len(matrix1[0]))] for i in range(len(matrix1))]
output:     
      if len(matrix1) != len(matrix2) or len(matrix1[0]) != len(matrix2[0]):
          return 'Matrices not compatible'
      
      return [[matrix1[i][j] + matrix2[i][j] for j in range(len(matrix1[0]))] for i in range(len(matrix1))]
  