code: void linkLast(final E e) {
      final E l = last;
      last = e;
  
      if (l == null) {
        first = e;
      } else {
        setNext(l, e);
        setPrevious(e, l);
      }
    }
docs: Links the element to the back of the deque so that it becomes the last element.
  
  @param e the unlinked element