input: def add_matrix(matrix1, matrix2):
      """
      Takes two matrices and returns their sum. If matrices are not of same dimension,
      returns 'Matrices not compatible'.
      Example:
      >>> add_matrix([[1,2],[3,4]], [[2,3],[4,5]])
      [[3, 5], [7, 9]]
      >>> add_matrix([[1,2,3],[3,4,5]], [[2,3],[4,5]])
      'Matrices not compatible'
      """
output:     if len(matrix1) != len(matrix2) or any(len(row1) != len(row2) for row1, row2 in zip(matrix1, matrix2)):
          return 'Matrices not compatible'
      result = []
      for row1, row2 in zip(matrix1, matrix2):
          result.append([a + b for a, b in zip(row1, row2)])
      return result