////   This file contains the C# code from Program 10.12 of//   "Data Structures and Algorithms//    with Object-Oriented Design Patterns in C#"//   by Bruno R. Preiss.////   Copyright (c) 2001--2002 by Bruno R. Preiss, P.Eng.  All rights reserved.////   http://www.brpreiss.com/books/opus6/programs/pgm10_12.txt//public class AVLTree : BinarySearchTree{    protected int height;    public override void AttachKey(object obj)    {	if (!IsEmpty)	    throw new InvalidOperationException();	key = obj;	left = new AVLTree();	right = new AVLTree();	height = 0;    }    // ...}