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RE: ISSUE-104: Several issues with the current treatment of disallowed vocabulary

From: Boris Motik <boris.motik@comlab.ox.ac.uk>
Date: Wed, 4 Jun 2008 09:58:36 +0100
To: "'Michael Schneider'" <schneid@fzi.de>, <public-owl-wg@w3.org>
Message-ID: <002301c8c621$2cffef50$7212a8c0@wolf>


I disagree with (B). If we were to go with this suggestion, then we would have two different types of OWL 2 DL ontologies:

- We'd have OWL 2 DL ontologies as specified in the SS document. These might actually use the reserved vocabulary, so they wouldn't
be serializable into RDF.
- We'd have OWL 2 DL ontologies that do not use the reserved vocabulary and therefore are serializable into RDF.

All of this seems unnecessarily complex and makes the whole story harder for people to follow. We need one place to say what
constitutes a valid OWL 2 DL ontology, and this definition should be in one place and not spread across the documents. True, we
might allow using the reserved vocabulary if we are not storing an ontology into RDF, but do we really need this/care about it? This
is particularly true if we work under the quite common assumption that RDF is the main syntax of OWL 2. It is therefore much more
coherent to just define everything in one place and not bother with the exceptions.

Also, I am not sure about (C). The whole idea of the reserved vocabulary is that it should be reserved -- that is, it is the
vocabulary that is given precise meaning by RDF and/or OWL semantics, so the users should "keep their hands off of it". It might be
the case that nothing bad happens even if we allow the usage of some particular URI, but why would we bother? What is the benefit of
doing this? Users will be more likely to clearly understand a statement "anything with the URI owl:* is reserved and you shouldn't
use it" than "this, that and the other is reserved, but those things are OK".

I accept that (A) might be an issue. Before we decide on the way forward, however, I would appreciate it is you could provide us
with a list of vocabulary that actually causes problems.



> -----Original Message-----
> From: Michael Schneider [mailto:schneid@fzi.de]
> Sent: 03 June 2008 21:38
> To: public-owl-wg@w3.org
> Cc: Boris Motik
> Subject: ISSUE-104: Several issues with the current treatment of disallowed vocabulary
> Hi!
> At the last telco, Boris (cc'ed) told me that ISSUE-104 (disallowed
> vocabulary) is probably moot, since there is already a notion of a "reserved
> vocabulary" in sec. 2.2 of the DL-Syntax document [1].
> I have read this section in the meanwhile, and have found three issues, which
> still remain. So I disagree that this issue is already resolved. The three
> issues are pretty independent of each other, and can therefore be discussed
> separately.
> I claim:
>   (A) Backwards Compatibility:
>       The set of disallowed vocabulary covers URIs,
>       which were allowed to be used in OWL 1 DL.
>       Hence, we have a backwards-compatibility issue.
>   (B) Responsibility:
>       The disallowed vocabulary should be in the
>       RDF mapping, not in the DL-syntax document.
>   (C) Relaxation:
>       It seems to be possible that a fraction of the
>       previously disallowed vocabulary can be allowed
>       to be used in OWL 2 DL.
> I will provide a more detailed argumentation for all these claims below.
> (A): Backwards compatibility
> ----------------------------
> The current disallowed (or "reserved") vocabulary is defined to be the set of
> all URIs within the RDF(S) and OWL namespace (and some additional). But in OWL
> 1 DL, according to [2], there existed some URIs from these namespaces, which
> were actually allowed to be used in OWL DL ontologies. An example is the RDF
> reification vocabulary.
> ! Proposal: Explicitly itemize the disallowed URIs, just as in the old AS&S.
> (B): Responsibility
> -------------------
> Technically, it doesn't matter whether the disallowed vocabulary is presented
> in the DL syntax or in the RDF mapping. However, there is a question of
> responsibility. I claim that having it in the DL syntax does not make much
> sense, while it there is a good reason to have it in the RDF mapping.
> >From the point of view of OWL 2 DL and its Functional Syntax, there is no
> reason to /not/ allow, for example, the use of the URI 'rdfs:subClassOf' at,
> for example, any position within a class assertion. This URI is neither used
> within the Functional Syntax itself, nor does it have a special meaning in the
> DL semantics.
> As an easy check, imagine that there was no RDF syntax for OWL DL. This would
> have no technical consequences for OWL DL, and the URIs from the RDF(S) and
> OWL vocabulary would then not be in any way special compared to any other URI.
> In fact, there would not even exist URIs in the OWL vocabulary.
> An exception are probably a few URIs like 'owl:Thing' or 'rdfs:comment', which
> actually play a special role within the OWL 2 DL syntax. But then it would not
> make sense to disallow /every/ URI from the RDFS and OWL namespace. Instead,
> these few URIs could be treated specifically, or could be replaced by new
> names.
> The only place where the disallowed vocabulary is really relevant is in the
> reverse RDF mapping. There, it might happen in some situations that the
> unrestricted usage of URIs from the RDF(S) or OWL vocabulary will lead to
> confusion when one applies the mapping rules, be it by making a reverse
> mapping impossible, or by leading to wrong results. This would then possibly
> break the "semantic roundtripping contract", which states that roundtripping
> from FS, through RDF and back to FS will always maintain the semantics of the
> original OWL DL ontology.
> ! Proposal: Move the definition of the disallowed vocabulary from the DL
> syntax document to the reverse RDF mapping.
> (C) Relaxation
> --------------
> The argument in (B), that allowing certain RDF(S) or OWL vocabulary might lead
> to technical problems with the reverse RDF mapping, cannot be used to disallow
> /every/ URI from RDF(S) and OWL. Instead, it should really be checked for each
> such URI whether it leads to problems with the reverse mapping or not.
> I had already some discussion on this topic for the concrete cases of rdf:List
> [3] and RDF reification [4] (though the use of the latter wasn't disallowed in
> OWL 1 DL, see (A)). And I think that in the analog way as discussed in these
> mails, several other URIs could be made accessible in OWL 2 DL.
> One has to check this, of course, and this probably demands some effort. And
> another question will be which of these URIs are of interest for custom usage
> at all. However, as I have heard several times, there was a desire to make
> more RDF graphs valid in OWL 2 DL. So here is a real chance to get more valid
> OWL DL ontologies in RDF graph form!
> Best,
> Michael
> [1] <http://www.w3.org/2007/OWL/wiki/Syntax#URIs_and_Namespaces>
> [2] <http://www.w3.org/TR/owl-semantics/mapping.html#4.2>
> [3] <http://lists.w3.org/Archives/Public/public-owl-wg/2008May/0188.html>
> [4] <http://lists.w3.org/Archives/Public/public-owl-wg/2008May/0190.html>
Received on Wednesday, 4 June 2008 09:00:10 UTC

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