/** 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 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 prev(Position p)
      throws InvalidPositionException, BoundaryViolationException {
    DNode v = checkPosition(p);
    DNode 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 Position insertBefore(Position p, Object element) 
      throws InvalidPositionException {			// 
    DNode v = checkPosition(p);
    DNode newNode = new DNode(v.getPrev(), v, element);
    return newNode;