////   This file contains the C++ code from Program 9.3 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_03.cpp//class PrePostVisitor : public Visitor{public:    virtual void PreVisit (Object&) {}    virtual void Visit (Object&) {}    virtual void PostVisit (Object&) {}};class PreOrder : public PrePostVisitor{    Visitor& visitor;public:    PreOrder (Visitor& v) : visitor (v)	{}    void PreVisit (Object& object)	{ visitor.Visit (object); }};class InOrder : public PrePostVisitor{    Visitor& visitor;public:    InOrder (Visitor& v) : visitor (v)	{}    void Visit (Object& object)	{ visitor.Visit (object); }};class PostOrder : public PrePostVisitor{    Visitor& visitor;public:    PostOrder (Visitor& v) : visitor (v)	{}    void PostVisit (Object& object)	{ visitor.Visit (object); }};