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Re: the idea of a 'reserved' vocabulary

From: Patrick Stickler <patrick.stickler@nokia.com>
Date: Sun, 16 Jun 2002 15:39:23 +0300
To: Pat Hayes <phayes@ai.uwf.edu>, Jonathan Borden <jonathan@openhealth.org>
CC: RDF Core <w3c-rdfcore-wg@w3.org>
Message-ID: <B93261AB.16DBC%patrick.stickler@nokia.com>

On 2002-06-14 3:08, "ext patrick hayes" <phayes@ai.uwf.edu> wrote:

> Here's a rough draft of what Id like to say in the RDF MT document
> about 'reserved' (we don't say 'dark' these says) vocabulary, to give
> you an idea of what is being proposed here.
> ------
> What does it mean to assert an RDF graph? The normal answer is that
> each triple can be read as a simple proposition, and the graph as a
> whole represents the conjunction of all of these propositions, so
> that what is asserted is the content of all the triples in the graph.
> Asserting a triple amounts to saying that it is true, and what that
> means, in turn, depends on what defines the meanings of the terms
> used in the graph. Before discussing that in more detail, we first
> note that it is also possible to use RDF triples simply as a
> data-structuring mechanism for encoding expressions of other
> languages which have a more complex syntax. If those 'encoding'
> triples are regarded as assertions in the same way as other triples,
> complexities can arise because the meaning they would have when seen
> simply as RDF assertions might not correspond to their intended
> interpretation in the other language. To accommodate such encodings
> and avoid these complications, we allow that some urirefs may be
> declared to be 'reserved'. Triples using urirefs from any reserved
> vocabulary can be present in an RDF graph but do not themselves make
> any RDF assertions. They may, however, be part of an encoding of
> expressions in some other language which itself may be asserted by
> the RDF graph in question, according to the semantic rules of that
> other language. We note that an RDF parser or processor is not
> required to treat such triples in any special way, unless it also
> needs to access the content expressed in that other language encoded
> in an RDF graph.

How does an RDF only processor know which URIrefs are reserved, so
that it knows which triples are not asserted?

Would we not need a fully generic mechanism to signal non-assertion?

> Since reserving a vocabulary effects the meaning of RDF, the
> authority to declare a uriref or urirefs 'reserved' in this sense
> rests with the W3C.  A uriref or set of urirefs is reserved only if
> it is declared to be so by a W3C Recommendation.

That seems a bit partisan to me. Are only W3C languages layered
on top of RDF allowed to be used with RDF? I hope not.

> In particular, 
> reserving a vocabulary cannot be done by simply asserting on a
> webpage that it is to be considered reserved. There is no way to
> state in RDF, or any language encoded in RDF, that a uriref is
> reserved, or for any RDF document to entail this as a consequence.

Bugger. Then every single SW application that only cares about
RDF assertions has to know *natively* which upper layer language
terms are reserved in order for it to determine the truth asserted
by an RDF graph?! Surely I must have missed something here, since
that is ... er, ahem (I'll be nice and not say out aloud)

> -----
> And then the basic MT rule for I(E) is slightly modified so that it reads:
> If E is a triple S P O . then I(E)=true if S, P and O are not reserved and
> ....
> -----
> That is all that is being suggested. And yes, this is the old
> 'unasserted triples' idea in a slightly updated form.

If it is not clear to all RDF applications using generic, vocabulary
netural means which triples are or are not asserted, then I oppose
this proposal on the grounds of it being completely impractical
and unscalable for real-world applications.

If I write an RDF application that encounters OWL "reserved"
vocabulary, I *refuse* to be required to intern special knowledge
of that reserved vocabulary in my application, which *never*
uses OWL terms, simply to know to ignore those triples and
not consider them asserted.

The distinction between asserted and unasserted triples must
be syntactic and explicit for *every* triple. It is not even
sufficient to define some class such as rdfs:Reserved and
declare each reserved URIref to be a member of that class, since
there is no guaruntee that a SW agent that recieves a triple
with such a "reserved" URIref will also possess the relevant class
membership knowledge for rdfs:Reserved.
So, if Lbase layering and unasserted triples cannot be captured
with a syntactic flag without introducing nonmonotonicity, then
it would appear that layering itself cannot be done monotonically.



Patrick Stickler              Phone: +358 50 483 9453
Senior Research Scientist     Fax:   +358 7180 35409
Nokia Research Center         Email: patrick.stickler@nokia.com
Received on Sunday, 16 June 2002 08:35:08 UTC

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