Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!hellgate.utah.edu!caen!math.lsa.umich.edu!zaphod.mps.ohio-state.edu!usc!apple!agate!ucbvax!pasteur!cory.Berkeley.EDU!achoi
From: achoi@cory.Berkeley.EDU (Andrew Choi)
Newsgroups: comp.lang.prolog
Subject: Is Prolog Limited To Only Horn Clauses/
Message-ID: <8956@pasteur.Berkeley.EDU>
Date: 16 Nov 90 06:38:33 GMT
Sender: news@pasteur.Berkeley.EDU
Reply-To: achoi@cory.Berkeley.EDU (Andrew Choi)
Organization: University of California, Berkeley
Lines: 29

Hi everyone.

I was convinced that Prolog can handle only Horn's Clauses
until a friend of mine brought me this example:

	Let's say we want to say:

		a ; b :- c.	(c -> (a or b))	/* Illegal in Prolog */

	According to rules of Boolean Algebra, we can re-express 
	proposition as:

		c -> (a or b)
		(not c) or (a or b)
		((not c) or b) or a
		(not (c and (not b))) or a
		(c and (not b)) -> a
		a :- c, not(b).		/* Legal in Prolog */

It seems like one can apply this procedure to all non-Horn's clauses
to make them Horn's Clauses.  Is this right?  Is there some mistakes
I don't see here?

Thanks for the help.


Name:  Andrew Choi	Internet Address:  achoi@cory.berkeley.edu
Tel:   (415)848-5658
#include <standard/disclaimer.h>