////   This file contains the C++ code from Program 6.11 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/pgm06_11.cpp//StackAsLinkedList::Iter::Iter (    StackAsLinkedList const& _stack) :    stack (_stack)    { Reset (); }bool StackAsLinkedList::Iter::IsDone () const    { return position == 0; }Object& StackAsLinkedList::Iter::operator * () const{    if (position != 0)	return *position->Datum ();    else	return NullObject::Instance ();}void StackAsLinkedList::Iter::operator ++ (){    if (position != 0)	position = position->Next ();}void StackAsLinkedList::Iter::Reset ()    { position = stack.list.Head (); }