// Textbook fragment 07.17
  /** Returns the parent of a node. */
  public Position<E> parent(Position<E> v)
    throws InvalidPositionException, BoundaryViolationException {
    BTPosition<E> vv = checkPosition(v);
    Position<E> parentPos = vv.getParent();
    if (parentPos == null)
      throw new BoundaryViolationException("No parent");
    return parentPos;
  }
  /** Returns an iterable collection of the children of a node. */
  public Iterable<Position<E>> children(Position<E> v)
    throws InvalidPositionException {
   PositionList<Position<E>> children = new NodePositionList<Position<E>>();
    if (hasLeft(v))
      children.addLast(left(v));
    if (hasRight(v))
      children.addLast(right(v));
    return children;
  }
  /** Returns an iterable collection of the tree nodes. */
  public Iterable<Position<E>> positions() {
   PositionList<Position<E>> positions = new NodePositionList<Position<E>>();
    if(size != 0)
      preorderPositions(root(), positions);  // assign positions in preorder
    return positions;
  }
 /** Returns an iterator of the elements stored at the nodes */
  public Iterator<E> iterator() {
    Iterable<Position<E>> positions = positions();
   PositionList<E> elements = new NodePositionList<E>();
   for (Position<E> pos: positions)
     elements.addLast(pos.element());
    return elements.iterator();  // An iterator of elements
  }
  /** Replaces the element at a node. */
  public E replace(Position<E> v, E o)
    throws InvalidPositionException {
    BTPosition<E> vv = checkPosition(v);
    E temp = v.element();
    vv.setElement(o);
    return temp;
  }