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Re: LC Comment on Entailment Regimes: Class and Property Variables take DL regime beyond FOL

From: Birte Glimm <birte.glimm@comlab.ox.ac.uk>
Date: Thu, 10 Nov 2011 10:14:05 +0100
Message-ID: <CABt65OfiGse3oyoPm-P0YprZjQEzuNUhQJsf1K7Zg4YTc0sVMA@mail.gmail.com>
To: Michael Schneider <schneid@fzi.de>
Cc: public-rdf-dawg-comments@w3.org
Michael,

Thank you for your comment about the SPARQL Entailment Regimes document.

Although the Direct Semantics Entailment Regime seems to allow for
higher order queries, the restriction that variables can only bind to
a finite set of names (such as class, property, or individual names)
means that even queries that seem to be higher order (as the subclass
query in the example in Sec. 6.4) can still be reduced to standard
first-order queries. In particular, one can answer such a query by
checking whether each of the finitely many instantiations of the query
(where class variables are replaced by class names from the queried
ontology, etc) are entailed by the queried ontology under OWL's Direct
Semantics.

We tried to make it clearer in Sec. 6.4 that no higher order reasoning
is required to answer SPARQL queries with the Direct Semantics
entailment regime. We do not think that discussing optimized
implementation techniques that go beyond entailment checking can be
discussed in the specification.

We hope that this answer addresses your comment and would be grateful
if you would acknowledge this by sending a reply to this mailing list.

Birte, on behalf of the SPARQL-WG

On 26 July 2011 13:02, Michael Schneider <schneid@fzi.de> wrote:
> Dear all!
>
> Document: SPARQL 1.1 Entailment Regimes
> State: LCWD
>
> The first paragraph in Chap. 6 "OWL 2 Direct Semantics Entailment Regime"
> states that variables are allowed in place of class names and property
> names. Nothing is said, however, about what the semantic consequences are of
> such enhanced flexibility.
>
> If one tries to conservatively extend the OWL 2 Direct Semantics [1] to
> support such queries, the only way I see is to do this by a translation into
> quantifying over variables for unary and binary relations (note that under
> the Direct Semantics, classes and properties are sets and binary relations,
> respectively). In other words, such a semantics goes beyond first-order
> logic (FOL). This is significant, as the OWL 2 Direct Semantics was designed
> as a fragment of standard FOL.
>
> I do not suggest to change the current specification, since I consider
> variables in class and property position to be useful. What I suggest is to
> make the "non-FOL issue" explicit in the document, to make implementers
> aware of it. I suggest to put such a discussion in Sec. 6.4, which is
> already dedicated to "Higher Order Queries", but does not treat this issue.
>
> [1] <http://www.w3.org/TR/2009/REC-owl2-direct-semantics-20091027/>
>
> Best regards,
> Michael
>
> --
> Dipl.-Inform. Michael Schneider
> Research Scientist, Information Process Engineering (IPE)
> Tel  : +49-721-9654-726
> Fax  : +49-721-9654-727
> Email: michael.schneider@fzi.de
> WWW  : http://www.fzi.de/michael.schneider
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>
>



-- 
Dr. Birte Glimm, Room 309
Department of Computer Science
University of Oxford
Parks Road
Oxford
OX1 3QD
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Received on Thursday, 10 November 2011 09:14:56 GMT

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