code: def apply(self, func, *extras):
          
          return self.__class__({key: func(value, *extras)
                                 for key, value in self.items()})
docs: >> a = AccumDict({'a': 1,  'b': 2})
          >> a.apply(lambda x, y: 2 * x + y, 1)
          {'a': 3, 'b': 5}