////   This file contains the C++ code from Program 9.1 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_01.cpp//class Tree : public virtual Container{    class Iter;public:    virtual Object& Key () const = 0;    virtual Tree& Subtree (unsigned int) const = 0;    virtual bool IsEmpty () const = 0;    virtual bool IsLeaf () const = 0;    virtual unsigned int Degree () const = 0;    virtual int Height () const;    virtual void DepthFirstTraversal (PrePostVisitor&) const;    virtual void BreadthFirstTraversal (Visitor&) const;    void Accept (Visitor&) const;};