docs: def set_difficulty(self, difficulty):
          """stub"""
          if not is_string(difficulty):
              raise InvalidArgument('difficulty value must be a string')
          if difficulty.lower() not in ['low', 'medium', 'hard']:
              raise InvalidArgument('difficulty value must be low, medium, or hard')
          self.my_osid_object_form._my_map['texts']['difficulty']['text'] = difficulty