// Textbook fragment 06.05
  /** Returns the number of elements in this list. */
  public int size();
  /** Returns whether the list is empty. */
  public boolean isEmpty();
  /** Returns the first node in the list. */
  public Position<E> first();
  /** Returns the last node in the list. */
  public Position<E> last();
  /** Returns the node after a given node in the list. */
  public Position<E> next(Position<E> p)
    throws InvalidPositionException, BoundaryViolationException;
  /** Returns the node before a given node in the list. */
  public Position<E> prev(Position<E> p)
    throws InvalidPositionException, BoundaryViolationException;
  /** Inserts an element at the front of the list, returning new position. */
  public void addFirst(E e);
  /** Inserts and element at the back of the list, returning new position. */
  public void addLast(E e);
  /** Inserts an element after the given node in the list. */
  public void addAfter(Position<E> p, E e)
    throws InvalidPositionException;
  /** Inserts an element before the given node in the list. */
  public void addBefore(Position<E> p, E e)
    throws InvalidPositionException;
  /** Removes a node from the list, returning the element stored there. */
  public E remove(Position<E> p) throws InvalidPositionException;
  /** Replaces the element stored at the given node, returning old element. */
  public E set(Position<E> p, E e) throws InvalidPositionException;