From: Jos de Bruijn <debruijn@inf.unibz.it>

Date: Tue, 17 Nov 2009 17:11:28 +0100

Message-ID: <4B02CB30.10302@inf.unibz.it>

To: Chris Welty <cawelty@gmail.com>

CC: kifer@cs.sunysb.edu, RIF WG Public list <public-rif-wg@w3.org>

Date: Tue, 17 Nov 2009 17:11:28 +0100

Message-ID: <4B02CB30.10302@inf.unibz.it>

To: Chris Welty <cawelty@gmail.com>

CC: kifer@cs.sunysb.edu, RIF WG Public list <public-rif-wg@w3.org>

> Unless anyone else has an opinion, Jos are you willing to make that change? > I updated the syntax and semantics of RIF-OWL DL combinations, as well as the embedding of RIF-OWL 2 RL combinations accordingly: - in class membership formulas a#b, b must be a constant symbol - in subclass formulas b##c, c,b must be constant symbols - subclass formulas may only occur as facts I imposed the last restriction, because I believe that in BLD-OWL 2 RL combinations, using subclass statements in rule bodies necessarily amounts to have been to introduce universal quantification in the body, making such combinations outside horn, which I think is undesirable. Besides, having subclass statements in rules without allowing variables does not seem all that useful. Best, Jos > > > Michael Kifer wrote: >> Yes, I agree that 3a is a reasonable fix. 3b is too big of a change, >> and I was >> not suggesting it for this round. >> >> michael >> >> >> On Tue, 10 Nov 2009 10:53:32 -0500 >> Chris Welty <cawelty@gmail.com> wrote: >> >>> I read this a little more carefully. >>> >>> Basically, the issue is whether to add some correspondence between >>> rif:subclass and rdf:subclass and between rif:type and rdf:type *in >>> the OWL compatibility section* of SWC. Such a correspondence is >>> already there for RDF compatibility, but Michael noted that it is not >>> "inherited" by the "OWL-DL" (now know as OWL Direct Semantics) >>> section. So, currently in SWC, the OWL-DL compatibility has no >>> correspondence between the rather obvious type/subclass relations in >>> the two languages. >>> >>> I agree this is a problem and should be fixed, and option #1 in >>> Michael's analysis, copied below (to leave it as is) is unacceptable. >>> >>> Option #2 is to just add a sentence to the text saying there is no >>> correspondence between owl and rif type/subclass. This is less than >>> satisfactory. >>> >>> Option #3 is to "fix" it somehow, and there are two variations there, >>> I'll call them 3a (just repeat the correspondences from RDFS in >>> OWL-DL) and 3b (do the best possible job mapping between owl and rif >>> subclass). >>> >>> </chair>I prefer option 3a. I agree with Jos' analysis of option 3b >>> and think it is too big a change.<chair> >>> >>> As chair, I am also willing to accept 2 or 3a as an oversight and bug >>> fix (I personally thought the correspondence between type and >>> subclass were "inherited" from the RDF correspondence, so 3a would >>> just make it the way I thought it was), however 3b seems to me, >>> procedurally, to be much more significant and requires a new last >>> call for SWC. >>> >>> -Chris >>> >>> Jos de Bruijn wrote: >>>>> In today's telecon I was asked to reanimate the issue of OWL >>>>> compatibility, >>>>> which was discussed 1 month ago. >>>>> >>>>> Here is the relevant message: >>>>> >>>>> http://lists.w3.org/Archives/Public/public-rif-wg/2009Sep/0017.html >>>>> >>>>> The current situation is a bug, IMO. If it isn't a bug then at >>>>> least that part >>>>> of the document is very unsatisfactory and obscure. Jos proposed 3 >>>>> solutions: >>>>> >>>>> 1- leave things as they are, assuming that # and ## are not of >>>>> interest >>>>> to users of RIF-OWL DL combinations >>>>> 2- explain the use of # and ## in the document (this would >>>>> certainly not >>>>> be a substantive change, so we should not run into procedural >>>>> problems) >>>>> 3- define the semantics of # and ## in RIF-OWL DL combinations in a >>>>> similar fashion as in RIF-RDF combinations: a one-to-one >>>>> correspondence >>>>> between # and OWL class membership statements and implication >>>>> between ## >>>>> and OWL subclassing. >>>>> >>>>> The easiest for him would be to do nothing (1), thus leaving things >>>>> unsatisfactory and obscure. His next choice is (3), which is also >>>>> my choice and >>>>> the "right thing to do." (3) stretches things a little, but it can >>>>> be argued >>>>> that it is a simple fix. >>>> In my earlier e-mail to Michael referred to I did not say what my >>>> preference is among the mentioned options. I guess arguments can be >>>> made for all three options, so in fact I do not have a strong >>>> preference, but I do have a concern about option (3): implementation >>>> might be harder. If, for example, implementation is done through >>>> embedding in other rules system, like the embedding of RIF-OWL2RL >>>> combination in the appendix of the document, quite a few rules need to >>>> be added for the ## construct. >>>> In particular, for every pair of distinct class names (A,B), we need to >>>> add the rule: >>>> >>>> Forall ?x (?x[rdf:type -> B] :- And(?x[rdf:type -> A] A##B)) >>>> >>>> This means adding a quadratic number of rules. >>>> >>>> Dealing with # is easy: in the mapping of RIF DL-document formulas to >>>> RIF documents [1] we simply map a#b to tr'(b)(a). Clearly, we would >>>> restrict b in formulas a#b to constant symbols. >>>> >>>> >>>> [1] >>>> http://www.w3.org/TR/rif-rdf-owl/#Embedding_RIF_DL-document_formulas_into_RIF_BLD >>>> >>>> >>>> >>>>> Solution (2) is more work. It fixes the obscurity aspect, not the >>>>> unsatisfactory aspect of the definitions. So, (3) seems like the >>>>> best way to >>>>> proceed. >>>>> >>>>> Solution (3) still leaves some problems, which are unrelated to the >>>>> above >>>>> issues. In the current semantics, subclassing in RIF implies >>>>> subclassing in >>>>> OWL/RDF, but not vice versa. >>>>> >>>>> In this regard, I would like to point to my follow-up message >>>>> http://lists.w3.org/Archives/Public/public-rif-wg/2009Sep/0019.html >>>>> Here I proposed a stronger semantics, which fixes this >>>>> non-entailment problem. >>>> Michael proposed the following semantics: >>>> >>>> {(A,B) | A rdfs:subclassOf B and A != B on the RDF side} >>>> = {(A,B) | A##B on the RIF side} >>>> >>>> I feel that this would take us out of Horn, even when considering >>>> Simple >>>> entailment, because implementation would require (classical) negation. >>>> At least, that is the only way I current see how this could be >>>> implemented. As we know, classical negation in the body amounts to >>>> disjunction in the head, so we would end up adding the following >>>> rule to >>>> the embedding of RDF-RDF combinations: >>>> >>>> Forall ?x, ?y (Or(?x##?y ?x=?y) :- ?x[rdfs:subClassOf -> ?y]) >>>> >>>> For RIF-OWL DL combinations such a semantics is even more problematic, >>>> because subclass in OWL DL means subset relation between class >>>> extensions, so the condition would look something like (X^C is the >>>> class >>>> extension of X): >>>> >>>> {(A,B) | A^C subset B^C and A^C != B^C on the RDF side} >>>> = {(A,B) | A##B on the RIF side} >>>> >>>> (Actually, we will need to apply some tricks here, since A and B are >>>> not >>>> constants on the OWL side, but I guess we can come up with a definition >>>> that kind-of achieves this semantics) >>>> >>>> A formula implementing the => direction of the condition for a pair of >>>> class names A,B would look something like (again, negation in the body >>>> becomes disjunction in the head): >>>> >>>> Forall ?x ( >>>> Or(A##B >>>> And(Forall ?x(?x[rdf:type -> B] :- ?x[rdf:type -> A]) >>>> Forall ?x(?x[rdf:type -> A] :- ?x[rdf:type -> B]))) >>>> :- >>>> Forall ?x(?x[rdf:type -> B] :- ?x[rdf:type -> A])) >>>> >>>> >>>> So, I would not be in favor of extending either the semantics of RDF or >>>> the semantics of OWL DL combinations with such a condition. >>>> >>>> >>>> Best, Jos >>>>> This would certainly be a substantive change semantically (although >>>>> not >>>>> significant textually). If we don't have the energy to do it this >>>>> time, >>>>> maybe for RIF 1.1. >>>>> >>>>> michael >>>>> >> > -- Jos de Bruijn debruijn@inf.unibz.it +390471016224 http://www.debruijn.net/Received on Tuesday, 17 November 2009 16:11:46 GMT

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