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From: geddis@polya.Stanford.EDU (Donald F. Geddis)
Newsgroups: comp.ai
Subject: Re: Request for opinions on AI textbook
Message-ID: <10429@polya.Stanford.EDU>
Date: 5 Jul 89 21:53:51 GMT
References: <295@kubix.UUCP> <65402@yale-celray.yale.UUCP>
Sender: Donald F. Geddis <geddis@polya.Stanford.EDU>
Reply-To: geddis@polya.Stanford.EDU (Donald F. Geddis)
Organization: Stanford University
Lines: 18

In article <65402@yale-celray.yale.UUCP> Duchier-Denys@cs.yale.edu (Denys Duchier) writes:
>For instance (NOT (P a)) can be replaced by (Q a)
>provided you add the horn clauses:
>
>		(OR (NOT (P x)) (Q x))
>		(OR (NOT (Q x)) (P x))

But this is equivalent to (P x) = (Q x).  What you want is (NOT (P x)) = (Q x),
which is
		(OR (P x) (Q x))		; ~P =>  Q, ~Q =>  P
		(OR (NOT (P x)) (NOT (Q x)))	;  P => ~Q,  Q => ~P

Unfortunately, the second of these is not a horn clause.

	-- Don Geddis
-- 
Geddis@Polya.Stanford.Edu
"There is no dark side of the moon, really.  Matter of fact, it's all dark."