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Re: completeness

From: Jim Hendler <hendler@cs.rpi.edu>
Date: Fri, 22 Feb 2008 09:48:05 -0500
Message-Id: <EBA19BBE-B2A0-45C9-9BCF-40CEA10696C7@cs.rpi.edu>
To: Michael Schneider <schneid@fzi.de>
Cc: Bijan Parsia <bparsia@cs.man.ac.uk>, OWL Working Group WG <public-owl-wg@w3.org>


Sent from my iPhone

On Feb 22, 2008, at 9:38, "Michael Schneider" <schneid@fzi.de> wrote:

> Bijan Parsia wrote:
>>> So, as you say, when bNodes are alternatively seen as (local) names
>>> instead
>>> of existentials, then a polytime decision algorithm exists for pD*
>>> entailment. Then, of course, pD* isn't a real RDFS extention
>>> anymore. So I
>>> wouldn't spec this, if pD* or a sublanguage is going to be
>> an OWL-Full
>>> fragment.
>> Why not? I point to the HTML5 design principles document (which
>> contains, I think, useful advice):
>> http://www.w3.org/TR/html-design-principles/#priority-of-constituencies
>> Fidelity to prior specs is *much* less important that actually
>> describing what implementors will do.
>> Oh, "as an OWL-Full fragment".
> Yes, that too. But I also talked about pD* as an "RDFS extention"  
> above. :)
>> Well, it's still a fragment of OWL-Full in the sense of having "if"  
>> and not
> "iff" semantics in pretty
>> much the way Pd* does for other constructs.
>>> But seeing bNodes as local names may of course become a typical
>>> non-standard restriction in reasoners for this language.
>> "Typical non-standard" seems like a "spec smell" to me.
> Ok, granted! Might be a misnomer, at least I don't like this notion,  
> too.
>> If it's typical, let's standardize it.
>> In fact, this is more than typical:
>> It's ubiquitous.
>>> And this might even
>>> be mentioned in an informative section of a spec. Just an idea.
>> [snip]
>> Though some people have tried to cast me otherwise, I'm really
>> eminently pragmatic: I prefer whatever it takes to make specs clear
>> and useful. I also prioritize clarity over general accessibility,
>> since the people who make the effort to understand can explain it to
>> others. But if the spec is unclear or ambiguous, then no one can
>> settle disputes.
> It wouldn't become ambiguous, since I suggested that there would be a
> normative part (existentials) separated from an informative part  
> (skolems as
> a meaningful alternative).
>> Similarly, specing things which no one implements is
>> a waste of time and can lower the perceived value of the spec (and
>> thus implementor efforts to adhere to the spec).
> I will only make a statement about the "specing complexity" here.
> For the WG, I think the least effort will be to simply have an OWL- 
> Prime
> with bNodes as existentials. For this to achieve, all we have to do is
> writing down a single sentence at the top of the OWL-Prime semantics  
> spec:
>  "This model-theoretic semantics for OWL-Prime
>  is an extension of the model-theoretic semantics for RDFS."
> together with a reference to
>  <http://www.w3.org/TR/rdf-mt/#DefSemanticExtension>
> This copycats the way how current OWL-Full is made a semantic  
> extention of
> RDFS. By this, all model-theoretic semantic conditions (and  
> indirectly all
> triple entailment rules) of RDFS automatically are inherited by OWL- 
> Prime.
> This includes, of course, the existential semantics of RDFS. We then  
> only
> need to define the remaining model-theoretic semantic conditions and  
> triple
> rules which are specific to OWL-Prime. And in the best case, we can  
> even
> simply take them from the pD* paper.
> If we instead were about switching to skolem semantics, then we would
> certainly need to do more work. We cannot just point to RDFS  
> semantics and
> say something like "this all, but without bNodes as existential".  
> This is
> not sufficient for a rigorous specification of OWL-Prime's semantics.
> Instead, we would essentially have to redefine the whole RDFS  
> semantics,
> both model-theoretic semantic conditions and triple rules, but this  
> time
> carefully checking that bNodes are now interpreted as skolems. This  
> may or
> may not be an easy task, I cannot say at the moment. It is certainly  
> a bit
> error-prone, and it would make the document for OWL-Prime a lot  
> larger.
> There would then be a lot of redundancy between two RDF related  
> semantics
> documents, the RDF(S) spec, and the OWL-Prime spec.
>> So I would put it the other way around: If what implementations need
>> and users want is bnodes as names, let's make *that* the specced
>> version. One can always provide informative text telling people how
>> it connects to OWL Full's semantics. However, I don't care, though I
>> strongly recommend against, making that bit normative as well as long
>> as implementations can clearly opt out of the part that they will opt
>> out of anyway.
> Another thing, which I would feel uneasy with, would be a situation,  
> where
> we have an RDFS with existential bNodes, an OWL-Prime with skolem  
> bNodes, an
> OWL-1.0-Full with existential bNodes, and an OWL-1.1-Full with  
> probably also
> existential bNodes in order to maintain backwards compatibility to  
> 1.0. It's
> not that I am a big fan of bNodes as existentials, that's really not  
> the
> case. But if this is going to change, then I would prefer to see it  
> changed
> for the whole family of RDF-based languages, not just for one or two  
> family
> members. Even if I was an implementor, this would really be more  
> confusing
> to me than bNodes as existentials, but then consistently over the  
> whole
> language family at least.
> Cheers,
> Michael
> --
> Dipl.-Inform. Michael Schneider
> FZI Forschungszentrum Informatik Karlsruhe
> Abtl. Information Process Engineering (IPE)
> Tel  : +49-721-9654-726
> Fax  : +49-721-9654-727
> Email: Michael.Schneider@fzi.de
> Web  : http://www.fzi.de/ipe/eng/mitarbeiter.php?id=555
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> us
Received on Friday, 22 February 2008 16:32:20 UTC

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