code: private E unlinkFirst(Node<E> f) {
          // assert f == first && f != null;
          final E element = f.item;
          final Node<E> next = f.next;
          f.item = null;
          f.next = null; // help GC
          first = next;
          if (next == null)
              last = null;
          else
              next.prev = null;
          size--;
          modCount++;
          return element;
      }
docs: Unlinks non-null first node f.