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Re: Rules WG -- draft charter -- NAF

From: Stefan Decker <stefan@ISI.EDU>
Date: Thu, 20 Nov 2003 19:19:56 +0000
Message-Id: <5.2.1.1.2.20031119103310.05cb6378@nitro.isi.edu>
To: "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>
Cc: www-rdf-rules@w3.org


Peter,

>Well, I am still confused as to how this would work.  How would one
>interpret a rule on an RDF graph without reference to the RDF (or some
>other) semantics?


It should take the RDF semantics into account.
But  compared to the OWL the semantics of RDF semantics is minimal.
But maybe I should first try to define what I mean by (extended) semantics 
of an RDF graph.
The following definition could serve as a starter:

Set T be the set of all triples.
An extended semantics esem is a mapping from pow(T) -> pow(T) such that
       sem(R) \subseteq R, R \in Pow(T).

So every extended semantics is itself monotonic :-)
E.g., RDF Schema semantics applied to an RDF graph delivers
only additional triples (e.g., additional subClassOf triples),
but would never remove anything.
Then a way to define CWA to extended semantics is by simply 
doing  CWA(rdfschema(R));
I think this works at least for some semantics - I don't think it works for 
all, but it would be interesting
to investigate where it works.

> > I was just talking about CWA on a graph.
>
>Hmm.
>
>What would the CWA make of the following graph?
>
>rdf:type rdf:type rdf:Predicate .
>rdf:type rdfs:domain rdfs:Resource .
>ex:john rdf:type ex:Person .
>Would it require that there be no domain element for, for example,
>rdfs:Class?
given a simple interpretation, yes - no domain element in RDFS:Class.
Given an RDF interpretation, the answer is still "no domain element", since
all that an RDF interpretation adds are the "axiomatic triples" -
(the RDF semantic conditions don't add anything).
Is this agreeable?

If yes, one could try to go one step higher on the semantics ladder and
investigate what an RDFS interpretation would add.
Taking the definition from above, one would try to compute the extended 
semantics
and then apply CWA to it. Do you see obvious problems with that?


>What would the CWA make of the folowing graph?
>
>ex:Student rdfs:subClassOf _:x .
>_:x rdf:type owl:Restriction .
>_:x owl:onProperty ex:sid .
>_:x owl:allValuesFrom xsd:integer .
Interpreting _:x as a skolem constant
and the rest as simple triple data I don't see the problem?


>In particular, would it require that there be no domain element for _:y in
>the following.
>
>_:y rdf:type owl:Restriction .
>_:y owl:onProperty ex:sid .
>_:y owl:allValuesFrom rdfs:Literal .
Again, just using an RDF interpretation from the model theory I don't see 
the problem.
Of course if I apply an OWL interpretation things look differently.
But maybe one should first start simple.

Am I missing anything?


>I forsee many similar problems in applying a CWA without use of semantics.

Could you explain them?

Best,
         Stefan



>[...]
>
>peter



--
http://www.isi.edu/~stefan
Received on Thursday, 20 November 2003 14:20:12 UTC

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