////   This file contains the C++ code from Program 9.19 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_19.cpp//void BinaryTree::DepthFirstTraversal (    PrePostVisitor& visitor) const{    if (visitor.IsDone ())	return;    if (!IsEmpty ())    {	visitor.PreVisit (*key);	left->DepthFirstTraversal (visitor);	visitor.Visit (*key);	right->DepthFirstTraversal (visitor);	visitor.PostVisit (*key);    }}