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Re: [TF-ENT] redundant blank nodes

From: Ivan Herman <ivan@w3.org>
Date: Thu, 08 Oct 2009 06:29:03 +0200
Message-ID: <4ACD6A8F.1080201@w3.org>
To: Birte Glimm <birte.glimm@comlab.ox.ac.uk>
CC: SPARQL Working Group <public-rdf-dawg@w3.org>
Hi Birte,

Birte Glimm wrote:
> Hi all,
> I am working my way through the open questions/issues, so the next on
> my list is something that Andy mentioned. At the moment, we have the 2
> conditions C1 and C2 that restrict the set of possible answers to a
> finite set of answers for RDF(S) entailment regimes. What these
> conditions don't cover is redundant answers that use different blank
> nodes. E.g.,
> suppose G is
> _:b1 :p :z.
> _:b2 :q :y.
> SG is:
> _:sg1 :p :z.
> _:sg2 :q :y.
> and the BGP of the query is
> ?x :p [] .
> We would get the two solutions
> (x, _:sg1)
> (x, _:sg2)
> because both _:sg1 :p [] and _:sg2 :p [] are well-formed RDF triples
> that are RDF(S) entailed by G (C1) each subject is in the set of terms
> used by the scoping graph and (C2) μ(?x) is a blank node occurring in
> SG. This always results in a finite answer sequence, but the more
> blank nodes we have origianlly, the more redundant answers we get.
> 
> Now what I would rather have only (x, _:sg1) as an answer. This could
> be defined by a notion of derivability I think. E.g.,
> Let R be a set of entailment rules for the entailment regime E, then,
> for each triple (s, p, o) in P(BGP), there must be a derivation of (s,
> p, o) from SG by means of R.
> Now I could use, for example, the RDFS entailment rules as suggested
> by ter Horst, and I get what I want because _:sg2 :p [] has no
> derivation.
> 
> What I am quite unhappy about is the use of "a set of entailment
> rules" because different systems might want to use different ways of
> deriving consequences and this might be too specific. Any opinions on
> that? Any better suggestion?
> 

To  add to your discomfort, and be extremely legalistic: the entailement 
rules in the RDF Semantics are explicitly stated as 'informative' in the 
RDFS document:-(

Ivan


> Birte
> 
> 
> 
> 

-- 

Ivan Herman, W3C Semantic Web Activity Lead
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Received on Thursday, 8 October 2009 04:25:56 GMT

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