code: public E pollFirst() {
          final Node<E> f = first;
          return (f == null) ? null : unlinkFirst(f);
      }
docs: Retrieves and removes the first element of this list,
  or returns {@code null} if this list is empty.
  
  @return the first element of this list, or {@code null} if
  this list is empty
  @since 1.6