////   This file contains the C++ code from Program 11.1 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/pgm11_01.cpp//class PriorityQueue : public virtual Container{public:    virtual void Enqueue (Object&) = 0;    virtual Object& FindMin () const = 0;    virtual Object& DequeueMin () = 0;};class MergeablePriorityQueue : public virtual PriorityQueue{public:    virtual void Merge (MergeablePriorityQueue&) = 0;};