code: def extend(self, iterable):
          
          new_right_list, new_left_list, extend_count = self._extend(self._right_list, self._left_list, iterable)
          return PDeque(new_left_list, new_right_list, self._length + extend_count, self._maxlen)
docs: Return new deque with all elements of iterable appended to the right.
  
          >>> pdeque([1, 2]).extend([3, 4])
          pdeque([1, 2, 3, 4])