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