////   This file contains the C++ code from Program 6.21 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_21.cpp//class Deque : public virtual Queue{public:    virtual Object& Head () const = 0;    virtual Object& Tail () const = 0;    virtual void Enqueue (Object&);    virtual void EnqueueHead (Object&) = 0;    virtual void EnqueueTail (Object&) = 0;    virtual Object& Dequeue ();    virtual Object& DequeueHead () = 0;    virtual Object& DequeueTail () = 0;};