// Textbook fragment 06.10
  /** Returns the number of elements in the list;  O(1) time */
  public int size() { return numElts; }
  /** Returns whether the list is empty;  O(1) time  */
  public boolean isEmpty() { return (numElts == 0); }
  /** Returns the first position in the list; O(1) time */
  public Position<E> first()
      throws EmptyListException {
    if (isEmpty())
      throw new EmptyListException("List is empty");
    return header.getNext();
  }
  /** Returns the position before the given one; O(1) time */
  public Position<E> prev(Position<E> p)
      throws InvalidPositionException, BoundaryViolationException {
    DNode<E> v = checkPosition(p);
    DNode<E> prev = v.getPrev();
    if (prev == header)
      throw new BoundaryViolationException
        ("Cannot advance past the beginning of the list");
    return prev;
  }
  /** Insert the given element before the given position, returning
    * the new position; O(1) time  */
  public void addBefore(Position<E> p, E element)
      throws InvalidPositionException {                 //
    DNode<E> v = checkPosition(p);
    numElts++;
    DNode<E> newNode = new DNode<E>(v.getPrev(), v, element);
    v.getPrev().setNext(newNode);
    v.setPrev(newNode);
  }