// Textbook fragment 06.11
  /** Insert the given element at the beginning of the list, returning
    * the new position; O(1) time  */
  public void addFirst(E element) {
    numElts++;
    DNode<E> newNode = new DNode<E>(header, header.getNext(), element);
    header.getNext().setPrev(newNode);
    header.setNext(newNode);
  }
  /**Remove the given position from the list; O(1) time */
  public E remove(Position<E> p)
      throws InvalidPositionException {
    DNode<E> v = checkPosition(p);
    numElts--;
    DNode<E> vPrev = v.getPrev();
    DNode<E> vNext = v.getNext();
    vPrev.setNext(vNext);
    vNext.setPrev(vPrev);
    E vElem = v.element();
    // unlink the position from the list and make it invalid
    v.setNext(null);
    v.setPrev(null);
    return vElem;
  }
  /** Replace the element at the given position with the new element
    * and return the old element; O(1) time  */
  public E set(Position<E> p, E element)
      throws InvalidPositionException {
    DNode<E> v = checkPosition(p);
    E oldElt = v.element();
    v.setElement(element);
    return oldElt;
  }