////   This file contains the C++ code from Program 16.12 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/pgm16_12.cpp//bool Digraph::IsCyclic () const{    CountingVisitor visitor;    TopologicalOrderTraversal (visitor);    return visitor.Count () != numberOfVertices;}