From: Michael Schneider <schneid@fzi.de>

Date: Tue, 22 Jul 2008 14:30:14 +0200

Message-ID: <0EF30CAA69519C4CB91D01481AEA06A0A26A91@judith.fzi.de>

To: "Ian Horrocks" <ian.horrocks@comlab.ox.ac.uk>

Cc: "Boris Motik" <boris.motik@comlab.ox.ac.uk>, <public-owl-wg@w3.org>

Date: Tue, 22 Jul 2008 14:30:14 +0200

Message-ID: <0EF30CAA69519C4CB91D01481AEA06A0A26A91@judith.fzi.de>

To: "Ian Horrocks" <ian.horrocks@comlab.ox.ac.uk>

Cc: "Boris Motik" <boris.motik@comlab.ox.ac.uk>, <public-owl-wg@w3.org>

Hi Ian! Ian Horrocks wrote on Monday, July 21, 2008: >Hi Michael, > >As I was explaining in my earlier emails, the suggested syntax >specification for OWL-R does not restrict the applicability of the >rules, Yes, I understand this. >it only provides a sufficiency condition for the semantic >guarantees currently described in Section 4.4 of the current Profiles >document; i.e., if an RDF graph satisfies the syntactic condition, >then the semantic guarantees hold. For graphs outside this set, the >rules will still derive many useful consequences. I have to admit that I am pretty lost. So it will be best to come up with a series of questions and comments. In your initial mail to Ivan [1] you write w.r.t. "conformance": """ we could define conforming OWL-R reasoners as those that, when presented with an RDF graph that can be parsed into the relevant structural spec, will satisfy the semantic conditions set out in Section 4.4 (http://www.w3.org/2007/OWL/wiki/Profiles#Relationship_between_OWL-R_DL_and_OWL-R_Full). """ But I do not understand how conformance can be defined based on the cited criterion. This criterion only talks about a certain semantic relationship between OWL-R-DL (or: the OWL-R specification) and the set of OWL-R rules. How are arbitrary implementations covered by this criterion? In particular such implementations, which are /not/ based on the OWL-R rule set? So, how can I apply the criterion in order to decide, whether a given implementation is conformant or not? It's perhaps an easy technical point which I overlook here, but I simply do not see it. I have some other issues with the criterion itself. On the one hand, there is the utility of the criterion: It seems to be pretty restricted by only taking assertion data into account. I am not sure whether this criterion is too weak for our purpose, in the sense that it might allow me to change the definition of either OWL-R-DL or OWL-R-Full to a significant degree, without making the stated relationship invalid. On the other hand, I am having trouble to check that the relationship stated by the criterion actually holds. The complicated definition of OWL-R-DL (the asymmetry of subClassOf axioms, etc.) makes it hard for me to see this easily. But these are just first thoughts, and I will need to do further checks on these questions, so let's defer a discussion about them for the moment. Another point which I wanted to make is on what you said in [1] was an "advantage" of the unification: """ * OWL-R implementations can provide additional functionality while still being fully OWL-R compliant because the extra functionality won't affect those graphs for which a guarantee of completeness is being made. """ This is something, which I do not really see RDF people regarding as an advantage, but rather as a mess. :) This sounds like, as if I can add arbitrary rules to the rule set given in the spec, and the result will still be a conformant OWL-R system. Or, in the most extreme case, as if I can produce a system, which simply produces all possible triples from all subject, predicate, and object nodes of triples in the base RDF graph. Even if the base RDF graph happens to be a syntactically valid OWL-R ontology in RDF graph form, the result would be "virtually everything". Calling such a "reasoner" to be OWL-R conformant is simply not a useful characterization from an RDF semantics point of view. But, perhaps, the notion of "OWL-R conformance" isn't really meant with such a vague and wide meaning. Perhaps, it is much more restricted by the criterion given in sec. 4.4. But this seems to lead back to my first question stated above, so I will wait for your answer, first, to get a better understanding. Cheers, Michael [1] <http://lists.w3.org/Archives/Public/public-owl-wg/2008Jul/0347.html> >It is easy to see that deriving *all* the consequences of the >equivalentClass axiom you describe would *in general* require non- >deterministic reasoning of a kind that implementations based on the >OWL-R rules will not provide. This kind of reasoning would obviously >cause an undesired increase in worst case complexity. > > >Regarding your second point, the reason for the relatively low >complexity of reasoning with ontologies that satisfy the syntactic >conditions is, intuitively, that they have a single canonical model >whose domain of interpretation consists exactly of those individuals >occurring in the ontology. The property axioms can thus be treated >simply as a set of rules that add implicit property assertions >(triples) -- which is exactly how they are implemented in the rule >set (See Table 4). > >Regards, >Ian > > > >On 19 Jul 2008, at 16:12, Michael Schneider wrote: > >> Hi Boris! >> >> On the last telco, we had this short discussion about to what >> degree the >> current definitions of OWL-R DL and OWL-R Full are aligned. As >> intended, I >> did a few checks afterwards, and here are my results and questions. >> >> >> (A) Restrictions on class related axioms >> ---------------------------------------- >> >> In OWL R DL, sub class axioms are asymmetrically specified: Not >> every class >> expression, which is allowed on the LHS of a sub class axiom is >> also allowed >> on the RHS, and vice versa. An example is that a unionOf class >> expression >> may only appear on the LHS. >> >> Other examples for class expressions, which are not allowed to >> appear on >> both sides of a sub class axiom are AllValuesFrom, SomeValuesFrom and >> (<=1)MaxCardinality. >> >> Other kinds of axioms which are also restricted in their use of >> certain >> class expressions are class equivalence and disjointness axioms, >> range and >> domain axioms, and class assertions. >> >> All these syntactic restrictions do not hold for OWL-R Full. For >> example, >> the following RDF graph is *not* a valid OWL-R DL ontology in RDF >> graph >> form, but it *is* a valid OWL R Full ontology: >> >> ex:C owl:equivalentClass _:x . >> _:x owl:unionOf ( ex:D1 ex:D2 ) . >> ex:w rdf:type ex:D1 >> ex:w rdf:type ex:D2 >> >> And this isn't even a particularly strange ontology from a DL point >> of view, >> i.e. it doesn't contain, for example, syntax reflection parts. In >> fact, it >> is a valid OWL 2 DL ontology in RDF graph form. >> >> Applying the OWL-R (Full) triple rules will result in: >> >> ex:w rdf:type ex:C >> >> Again, this is not a weird result, but one which one would expect >> from OWL 2 >> DL, too. And the entailed triple is of course syntactically valid >> in OWL-R >> DL. >> >> If the unification process is performed without a change of either >> the OWL-R >> (DL) syntactic restrictions, or the OWL-R (Full) rules, then the >> set of >> OWL-R rules will produce such additional "DL-meaningful looking" >> results >> from "DL-meaningful looking" RDF graphs, which will go beyond the >> OWL-R >> specification. >> >> >> (B) Unrestricted property related axioms >> ---------------------------------------- >> >> In the telco, I specifically asked for sub property chains as an >> example for >> a language feature, which is in OWL-R Full, but not in OWL-R DL. >> However, I >> now see that sub property chains are really included in OWL-R DL. >> On the one >> hand, property expressions are unrestricted: >> >> 4.2.2 Property expressions >> >> "Property expressions in OWL-R DL are identical to the >> property expressions in OWL 2 [OWL 2 Specification]. >> >> And further, property axioms are also unrestricted: >> >> 4.2.5 Axioms >> >> OWL-R DL redefines all of [OWL 2 Specification] that refer to >> ClassExpression. >> [...] >> All other axioms in OWL-R DL are defined as in OWL 2. >> >> But I wonder how this can be the case. Is it really certain that the >> unrestricted use of all the property axioms will maintain >> tractability? >> Again, my test case would be sub property chains here. >> >> Cheers, >> Michael >> >> -- >> Dipl.-Inform. Michael Schneider >> FZI Forschungszentrum Informatik Karlsruhe >> Abtl. Information Process Engineering (IPE) >> Tel : +49-721-9654-726 >> Fax : +49-721-9654-727 >> Email: Michael.Schneider@fzi.de >> Web : http://www.fzi.de/ipe/eng/mitarbeiter.php?id=555 >> >> FZI Forschungszentrum Informatik an der Universität Karlsruhe >> Haid-und-Neu-Str. 10-14, D-76131 Karlsruhe >> Tel.: +49-721-9654-0, Fax: +49-721-9654-959 >> Stiftung des bürgerlichen Rechts >> Az: 14-0563.1 Regierungspräsidium Karlsruhe >> Vorstand: Rüdiger Dillmann, Michael Flor, Jivka Ovtcharova, Rudi >> Studer >> Vorsitzender des Kuratoriums: Ministerialdirigent Günther Leßnerkraus >> -- Dipl.-Inform. Michael Schneider FZI Forschungszentrum Informatik Karlsruhe Abtl. Information Process Engineering (IPE) Tel : +49-721-9654-726 Fax : +49-721-9654-727 Email: Michael.Schneider@fzi.de Web : http://www.fzi.de/ipe/eng/mitarbeiter.php?id=555 FZI Forschungszentrum Informatik an der Universität Karlsruhe Haid-und-Neu-Str. 10-14, D-76131 Karlsruhe Tel.: +49-721-9654-0, Fax: +49-721-9654-959 Stiftung des bürgerlichen Rechts Az: 14-0563.1 Regierungspräsidium Karlsruhe Vorstand: Rüdiger Dillmann, Michael Flor, Jivka Ovtcharova, Rudi Studer Vorsitzender des Kuratoriums: Ministerialdirigent Günther LeßnerkrausReceived on Tuesday, 22 July 2008 12:30:58 UTC

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