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Re: [TF-ENT] Entailment regimes doc update

From: Ivan Herman <ivan@w3.org>
Date: Wed, 17 Feb 2010 13:30:52 +0100
Message-ID: <4B7BE17C.2040005@w3.org>
To: Birte Glimm <birte.glimm@comlab.ox.ac.uk>
CC: SPARQL Working Group <public-rdf-dawg@w3.org>, Chimezie Ogbuji <ogbujic@ccf.org>
Mini editorial issues:

- "query answer: in:" (section 2.2., right before the table for the
simple results

- Better use the same formulation everywhere. In, say, RDFS, (C2) is

[[[
(C2) For each variable x in V(BGP), sk(μ(x)) occurs in sk(SG).
]]]

whereas, for OWL2/RDF it says

[[[
(C2) Each variable x that occurs in a triple in BGP is such that
sk(μ(x)) occurs in sk(SG).
]]]

meaning, I believe, the same, but let us use the same terms:-)

- "ontolgy" -> "ontology" in the section referreing to scm-eqc2

More substantial comment:

I must admit I am a bit bothered by 5.2.1. I understand what is
happening but I am bothered anyway:-( I am afraid that it will become
fairly difficult for an average OWL RL user to follow what is happening
(and an average OWL RL user may not understand all the details and
intricacies of all this). Eg, if I look at

SELECT ?r WHERE { ex:a ?r ex:a }

for the data

[[
ex:a ex:b ex:c .
ex:w ex:ww 123 .
ex:z ex:zz 123 .
]]

then ?r/owl:sameAs will be the result. This is because the dt-eq will
bring owl:sameAs into the picture through the back door of the rule set.
As a consequence, eq-ref will produce the (ex:a owl:sameAs ex:a) triple
and due to dt-eq (C2) will not apply. On the other hand, with the data

[[
ex:a ex:b ex:c .
]]

(C2) will apply, and the result set will be empty. This is true
mathematically, but very counter-intuitive for a user:-(

I am not sure how to solve that, I must admit. One solution would be to
say that the axiomatic triples listed in[1] are implicitly part of the
graph, beyond your reference to[2]. In contrast to the RDFS axiomatic
triples [1] is finite triple set and therefore does not produce infinite
results. On the other hand, it takes away the peculiarities of the
consequences of (C2) because it will list essentially all OWL terms.
However, [1] is marked as informative in the RDF based semantics text:-(
(I remember having had long discussions on that in the OWL 2 WG...).
Also, this may bring in a sh...load of extra results that one would not
want to see...

Sigh.

Ivan

[1]
http://www.w3.org/TR/2009/REC-owl2-rdf-based-semantics-20091027/#Appendix:_Axiomatic_Triples_.28Informative.29
[2]
http://www.w3.org/TR/2009/REC-owl2-syntax-20091027/#Entity_Declarations_and_Typing



On 2010-2-16 18:42 , Birte Glimm wrote:
> Hi all,
> I have committed a new version of the entailment regimes document:
> http://www.w3.org/2009/sparql/docs/entailment/xmlspec.xml
> 
> There is now a description of the OWL RDF-Based Semantics incl. the
> OWL 2 RL profile. The OWL 2 RL profile can also be used with Direct
> Semantics, so I have added that there too. Further I have added a
> section about aggregates with RDF(S) entailment, addressing at least
> parts of Axel's comments (no owl:sameAs discussion yet for
> aggregation). I also defined the behaviour for inconsistent graphs
> more clearly because the previous spec didn't define the scoping graph
> in the case of inconsistencies. It was rather assumed that the scoping
> graph is still equivalent to the active graph, so that systems can
> just use the graph as is modulo bnode renaming, but that allowed
> infinite answers for inconsistent graphs. I now use Axel's suggestion
> for condition C2 and require not only bindings for variables inn
> subject position to occur in the input, but require this for all
> variables. This also solves the OWL RDF-Based semantics problem where
> you can have infinite answers from owl:topDataProperty, which relates
> an individual to all data values. Now all RDF-Based regimes (RDF,
> RDFS, OWL 2 RDF-Based (for OWL Full and OWL RL)) use the same
> definitions, which is nice IMO.
> 
> Birte
> 
> 

-- 

Ivan Herman, W3C Semantic Web Activity Lead
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Received on Wednesday, 17 February 2010 12:28:24 GMT

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