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[ontolog-forum] named graphs and rdfg:subGraphOf

To: semantic-web@xxxxxx, "[ontolog-forum]" <ontolog-forum@xxxxxxxxxxxxxxxx>
From: Jakub Kotowski <jakubkotowski@xxxxxxx>
Date: Wed, 07 Oct 2009 22:46:27 +0200
Message-id: <4ACCFE23.5050205@xxxxxxx>
Hello,    (01)

I am trying to understand the definition of the rdfg:subGraphOf property
from [1,2]. It says that:    (02)

<f,g> is in IEXT(I(rdfg:subGraphOf))  iff
rdfgraph(f) is a subset of rdfgraph(g)    (03)

What confuses me is probably the "syntactic part" of the definition:
rdfgraph(f) is the ("syntactical") set of triples of the named graph f.
I am wondering whether it means that if rdfgraph(g) does not contain all
the triples from rdfgraph(f) I can infer them - so that after the
inference proces I'll get a new, enriched rdfgraph(g) for which the
condition already holds (rdfgraph(f) is a subset of rdfgraph(g))...?
Instead of this description I would almost rather like to ask whether it
means that the triple f rdfg:subGraphOf g entails:
g {
        plus whatever was in g before
But that doesn't seem to be correct because entailment is not defined
over a set of named graphs.    (04)

The alternative would be that a knowledge base containing:
        the triple              f rdfg:subGraphOf g
        the named graph         f
        the named graph         g ...(the original, not enriched one)    (05)

is inconsistent because rdfgraph(f) is not a subset of rdfgraph(g) but
the triple f rdfg:subGraphOf g is asserted. Well, this alternative maybe
does not even make sense because a set of (accepted) named graphs is
defined to have the usual RDF semantics of the merge of the respective
graphs.    (06)

On the one hand the first interpretation would seem more plausible
because it would make the rdfg:subGraphOf property usable as a way of
nesting named graphs, on the other hand, the provenance motivation for
named graphs seems to be in favour of the alternative interpretation
which sees the property as rather descriptive. After all, if a graph
changes because of inference (inferred triples are added) then the new
graph should probably be associated with provenance information which
documents that it was inferred and possibly how.    (07)

At least the following is true, right?    (08)

:f rdfg:subGraphOf :g does not hold if :f and :g are specified as follows:    (09)

:f {
        :a rdfs:subclassOf :c .
}    (010)

:g {
        :a rdfs:subclassOf :b .
        :b rdfs:subclassOf :c .
}    (011)

Am I just making things too complicated and overlooking something?
By the way, esentially this question has already been asked but
unfortunately left unanswered:
http://lists.w3.org/Archives/Public/semantic-web/2006May/0108.html    (012)

Best regards,
Jakub Kotowski    (013)

[1] Named graphs, provenance and trust Export
Jeremy J. Carroll, Christian Bizer, Pat Hayes, Patrick Stickler
In WWW '05: Proceedings of the 14th international conference on World
Wide Web (2005), pp. 613-622.    (014)

[2] Named graphs
J. Carroll, C. Bizer, P. Hayes, P. Stickler
Web Semantics: Science, Services and Agents on the World Wide Web In
World Wide Web Conference 2005------Semantic Web Track, Vol. 3, No. 4.
(December 2005), pp. 247-267.    (015)

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