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