////   This file contains the C++ code from Program 10.14 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/pgm10_14.cpp//void MWayTree::DepthFirstTraversal (    PrePostVisitor& visitor) const{    if (!IsEmpty ())    {	for (int i = 0; i <= numberOfKeys + 1; ++i)	{	    if (i >= 2)		visitor.PostVisit (*key [i - 1]);	    if (i >= 1 && i <= numberOfKeys)		visitor.Visit (*key [i]);	    if (i <= numberOfKeys - 1)		visitor.PreVisit (*key [i + 1]);	    if (i <= count)		subtree [i]->DepthFirstTraversal (visitor);	}    }}