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Re: mergeg in current Semantics ED

From: Peter Patel-Schneider <pfpschneider@gmail.com>
Date: Thu, 7 Mar 2013 14:26:27 -0800
Message-ID: <CAMpDgVzaePmsebDNv-PnX0NA0m8EUsozLQMEVZ7DrGhVChAoEQ@mail.gmail.com>
To: Antoine Zimmermann <antoine.zimmermann@emse.fr>
Cc: RDF WG <public-rdf-wg@w3.org>
Under the new regime, b-node sharing depends on blank node scope.  (A bit
on this probably needs to be added to Concepts.)

If the two graphs are in the same scope, then `b' is the "same" node, so
the entailment does not follow, as expected.  I see that the merge lemma
needs to be rewritten, because this case should be excluded.

If the two graphs are in different scopes then they cannot share a blank
node.  This is mentioned in Semantics - I suggested that it be mentioned
more prominently. So, if the two graphs are from different scopes the
situation you provide cannot happen, and then the merge lemma goes forward.

Reasoners always work with concrete representations, so they need to
"rename" b-nodes if necessary when the graphs come from different scopes,
but if the graphs are in the same scope then they should not do any
"renaming".

The idea of b-node scope was added to handle existing cases where b-nodes
are shared between graphs.

peter

On Thu, Mar 7, 2013 at 7:52 AM, Antoine Zimmermann <
antoine.zimmermann@emse.fr> wrote:

> There is a problem with the definition of merge in the draft.
>
> I'm using math notations instead of a concrete serialisation syntax
> because I want to show things at the abstract syntax level, which is what
> RDF Semantics relies on.
>
> Let us take a blank node b from the set of blank nodes. Let us consider
> the two graphs G1 = {(<s1>,<p1>,b)} and G2 = {(<s2>,<p2>,b)}.
>
> Let us ask ourselves whether {G1,G2} entails:
>
> G = {(<s1>,<p1>,b),(<s2>,<p2>,b)}
>
> The answer is trivially NO wrt the current semantics of the ED.
> Yet, reasoners typically would, in this case, merge G1 and G2 to get a
> single graph and work out the non-entailment. However, with the new
> definition of merge, this breaks and fails. Reasoners must standardise
> apart the bnodes and that's what the merge should be.
> --
> Antoine Zimmermann
> ISCOD / LSTI - Institut Henri Fayol
> École Nationale Supérieure des Mines de Saint-Étienne
> 158 cours Fauriel
> 42023 Saint-Étienne Cedex 2
> France
> Tél:+33(0)4 77 42 66 03
> Fax:+33(0)4 77 42 66 66
> http://zimmer.**aprilfoolsreview.com/<http://zimmer.aprilfoolsreview.com/>
>
>
Received on Thursday, 7 March 2013 22:26:55 GMT

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