////   This file contains the C++ code from Program 9.14 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/pgm09_14.cpp//bool NaryTree::IsEmpty () const    { return key == 0; }Object& NaryTree::Key () const{    if (IsEmpty ())	throw domain_error ("invalid operation");    return *key;}void NaryTree::AttachKey (Object& object){    if (!IsEmpty ())	throw domain_error ("invalid operation");    key = &object;    subtree.SetLength (degree);    for (unsigned int i = 0; i < degree; ++i)	subtree [i] = new NaryTree (degree);}Object& NaryTree::DetachKey (){    if (!IsLeaf ())	throw domain_error ("invalid operation");    Object& result = *key;    key = 0;    for (unsigned int i = 0; i < degree; ++i)	delete subtree [i];    subtree.SetLength (0);    return result;}