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From: kilian@seas.gwu.edu (Jens Kilian)
Newsgroups: comp.lang.prolog
Subject: Re: '->' operator
Message-ID: <2497@sparko.gwu.edu>
Date: 4 Jan 91 14:47:02 GMT
References: <291@valverde.cs.utexas.edu> <1991Jan4.030918.24996@athena.cs.uga.edu> <16491@cs.utexas.edu> <1991Jan4.061142.26941@athena.cs.uga.edu>
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Reply-To: kilian@seas.gwu.edu (Jens Kilian)
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As far as I know, the '->' operator is implemented as a 'local cut'. Thus,

	P -> Q; R

is equivalent to 

	P, !, Q; R

provided that the scope of the cut is limited to the disjunction. According
to the Quintus manual,  (P -> Q) is equivalent to (P -> Q; false), which
explains why (P -> Q) fails if P fails. The only reason I can imagine is
that (P -> Q; false) need not create a choice point, while (P -> Q; true)
must create one.
--
Internet: kilian@seas.gwu.edu		SnailMail: 4715 MacArthur Blvd.
(I don't know any other addresses ...)		   Washington, DC 20007
"Sie hawwe-mer so e bekannt Physionomie, ich mahn, ich misst Ihne kenne.
 Sinn-Se net, um Vergebung, der Herr Assesser Ranft ?"