- From: Axel Polleres <axel.polleres@deri.org>
- Date: Thu, 28 Jun 2007 09:59:23 +0100
- To: Gary Hallmark <gary.hallmark@oracle.com>, "Public-Rif-Wg (E-mail)" <public-rif-wg@w3.org>
Gary Hallmark wrote: > > > > Axel Polleres wrote: > >> As for adding (locally) stratified negation to horn, I can say that >> ignoring rules with stratified negation would simply give you >> incomplete models, but all >> inferred atoms would still be sound. >> If I understood correctly, Sandro was with his nnotion of "impact" >> talking about different treatments of "fallback", e.g. something like >> (this probably only applies to deductive rules though): >> >> a) ignoring X will lead to sound inferences only but inferences >> might be incomplete >> b) ignoring Y will lead preserve completeness but unsound inferences >> might arise >> c) ignoring z will neither preserve soundness nor completeness >> >> etc. >> while the latter two are probvably pointless, I could well imagine use >> cases for a) e.g. when doing query answering over a rule base >> I might be happy to get sound answers but not requiring all answers >> >> example: >> >> p(a) <- p(b). >> p(b). >> q(c). >> r(d). >> >> p(X) <- not q(X). >> >> When I now ask for >> p(X). >> >> I'd get p(a) and p(b) in all cases even if I ignore the last rule. >> p(c) would only get inferred when having the last rule. > > > actually you'll get p(a) twice rather than p(c). I wasn't thinking of multiset-semantics. > But I see your point. My points are > 1. you need quite a bit of detail in the fallback mechanism to know that > not handling "not" must fallback to ignoring the containing *rule*. yes, you'd need to describe which syntactic "components" (in this case you might drop) and what the effect would be (in this case sound but incomplete inferences) > Just ignoring the *term* containing "not" would be very bad. yes, you can only ignore the whole rule - all or nothing. > 2. this fallback doesn't work in the presence of some extensions (like > classical negation) I never claimed that. classical negation is a pretty much different issues, see: http://www.w3.org/2005/rules/wg/wiki/negation > and would be ill-advised (though technically sound, > I guess) in the presence of "world-closing" extensions like aggregation not sure what you mean here... -- Dr. Axel Polleres email: axel@polleres.net url: http://www.polleres.net/
Received on Thursday, 28 June 2007 09:00:23 UTC