- From: Chris Welty <cawelty@gmail.com>
- Date: Tue, 10 Nov 2009 10:53:32 -0500
- To: Jos de Bruijn <debruijn@inf.unibz.it>
- CC: kifer@cs.sunysb.edu, RIF WG Public list <public-rif-wg@w3.org>

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 >> > -- Dr. Christopher A. Welty IBM Watson Research Center +1.914.784.7055 19 Skyline Dr. cawelty@gmail.com Hawthorne, NY 10532 http://www.research.ibm.com/people/w/welty

Received on Tuesday, 10 November 2009 15:54:17 UTC