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		 Breadth-First Queue Data Structures

			Fernando C. N. Pereira

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COPYRIGHT NOTICE, LICENSE AND DISCLAIMER.

Copyright 1987 by Fernando C. N. Pereira

Permission to use, copy, modify, and distribute this software and its
documentation for any purpose and without fee is hereby granted,
provided that the above copyright notice appear in all copies and that
both the copyright notice and this permission notice and warranty
disclaimer appear in supporting documentation, and that the name of
the author, Fernando C. N. Pereira, not be used in advertising or
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written prior permission.

The author disclaims all warranties with regard to this software,
including all implied warranties of merchantability and fitness.  In
no event shall the author be liable for any special, indirect or
consequential damages or any damages whatsoever resulting from loss of
use, data or profits, whether in an action of contract, negligence or
other tortious action, arising out of or in connection with the use or
performance of this software.


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:- mode start_queue(-).

start_queue(Empty-Empty).

:- mode empty_queue(+).

empty_queue(Front-_Back) :- var(Front).

:- mode dequeue(+,-,-).

dequeue(Front-Back,Element,Rest-Back) :-
   nonvar(Front),
   Front = [Element|Rest].

:- mode enqueue(+,+,-).

enqueue(Front-[Element|Back],Element,Front-Back).

:- mode enqueue_list(+,+,-).

enqueue_list([],Queue,Queue).
enqueue_list([Item|Items],Queue0,Queue) :-
   enqueue(Queue0,Item,Queue1),
   enqueue_list(Items,Queue1,Queue).