////   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) 1998 by Bruno R. Preiss, P.Eng.  All rights reserved.////   http://www.pads.uwaterloo.ca/Bruno.Preiss/books/opus4/programs/pgm10_12.cpp//void AVLTree::AttachKey (Object& object){    if (!IsEmpty ())	throw domain_error ("invalid operation");    key = &object;    left = new AVLTree ();    right = new AVLTree ();    height = 0;}Object& AVLTree::DetachKey (){    height = -1;    return BST::DetachKey ();}