code: def append(self, elem):
          
          new_left_list, new_right_list, new_length = self._append(self._left_list, self._right_list, elem)
          return PDeque(new_left_list, new_right_list, new_length, self._maxlen)
docs: Return new deque with elem as the rightmost element.
  
          >>> pdeque([1, 2]).append(3)
          pdeque([1, 2, 3])