- From: Stasinos Konstantopoulos <konstant@iit.demokritos.gr>
- Date: Thu, 6 Dec 2007 16:26:38 +0200
- To: public-powderwg@w3.org
Dear POWDER, just my couple of cents on the URI groups issue. (This has nothing to do with the equivalence vs subsumption thread.) Andrea's proposal (current URI group document) looks like this: <owl:Class rdf:ID="ResourceOnExampleDotOrg"> <owl:equivalentClass> <owl:Restriction> <owl:onProperty rdf:resource="&wdr;includeHost" /> <owl:hasValue>example.org</owl:hasValue> </owl:Restriction> </owl:equivalentClass> </owl:Class> (it doesn't really matter if it's going to be an implication or an equivalence, the latter simply promises that there is no other way to be in the ResourceOnExampleDotOrg class, like a DNS alias or something.) Now, I have already voiced my concerns about this approach on telcos and emails, and I think I should re-iterate it, just for clarity's sake. In a nutshell, it a well-known fact that once concrete domains are added (strings, regexps, integers, and the like) it get really hard to come up with a complete deductive operator and the RDF graphs cannot be closed (ie, automatically generate the graph where all implicit facts have been made explicit.) In other words, there are important axioms defining the domain of URIs that cannot be expressed in OWL, e.g. that a resource with value "X.Y.Z" for its "includeHost" property also has value "Y.Z" and "Z" for the same property. The domain of URIs cannot be closed under OWL, because OWL is not expressive enough to imply property values given other property values. For this reason, the fact that ResourceOnExampleDotOrg has been axiomatically defined in our example here isn't of much use, as it pretends to specify all and every instances that are in the class, where, in fact, it cannot. My proposal is that URI Groups are defined in pretty much the same way as reification: they're there, but not within the logic. Something along the lines of: <owl:Class rdf:about="#FOSIchildSafe"> <rdfs:subClassOf rdf:ID="CSassertion" rdf:resource="#ChildSafe"/> </owl:Class> <rdf:Statement rdf:about="#CSassertion"> <foaf:maker rdf:resource="http://www.example.org/foaf.rdf#me"/> </rdf:Statement> <rdf:Statement rdf:about="#CSassertion"> <wdr:includeHost "example.org"^^xsd:string/> </rdf:Statement> This way we say nothing about subsumption or equivalenc and the such, and an external, POWDER-specific tool will go over the DR and explicitly add a resource to the FOSIchildSafe class, if appropriate. OWL subsumption will then say that such a resurce is also ChildSafe. BTW, with OWL semantics out of the way, one can possibly even go back and throw excetions and excludeHost statements back into the game. Hope this helps a bit, stasinos
Received on Thursday, 6 December 2007 14:27:01 UTC