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From: pereira@sri-unix.UUCP
Newsgroups: net.sources
Subject: Prolog library: flagro.pl
Message-ID: <97@sri-unix.UUCP>
Date: Mon, 15-Aug-83 01:52:16 EDT
Article-I.D.: sri-unix.97
Posted: Mon Aug 15 01:52:16 1983
Date-Received: Wed, 10-Aug-83 02:12:14 EDT
Lines: 46

/* FLAGRO.PL : Flag handling

						UTILITY
						Lawrence
						Updated: 30 March 81
*/

	%%%  Compile this module
	%%%  FLAGRO requires no other modules



  /* EXPORT */

  :- public flag/3.



  /* MODES */

	:- mode flag(+,?,?).



% Flags are stored in the database keyed under the Flag itself with
% the information packaged into a functor as follows:
%
%		Flag --> '$flag'(Flag,Value)
%
% This routine maintains these flags returing previous values as Old and
% updating the flag to New each time. The code actually checks to see if
% this updating really requires changing the database.
%


  flag(X,Old,New)
	:- nonvar(X),
	   ( recorded(X,'$flag'(X,O),ID)  ;  O = 0 ),
	   ( Old == New,
	     !,
	     Old = O	    ;	!,
				Old = O,
				( nonvar(ID), erase(ID)  ;  true ),
				recorda(X,'$flag'(X,New),_)
	   ),
	   !.