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Re: Action-67 some examples on b-nodes issues and their impact on users

From: Ivan Herman <ivan@w3.org>
Date: Sat, 26 Jan 2008 10:13:28 +0100
Message-ID: <479AF9B8.3010703@w3.org>
To: "Jeff Z. Pan" <jpan@csd.abdn.ac.uk>
CC: public-owl-wg@w3.org

Jeff Z. Pan wrote:
> Ivan,
>> thanks a lot. These types of down-to-Earth examples help me at least 
>> (modulo Peter's comment that I still have to digest:-).
>> In trying to grasp and understand the consequences of the 'existential 
>> vs. skolem' proposal/discussion, I must admit that I found some gaps 
>> in my own understanding. I have therefore some questions; if you, 
>> Boris, Bijan, Jeremy, or the others could answer those it would help 
>> me at least....
>> 1. Scope of skolemization
>> I am not sure I fully understand the proposal in terms of the 'skope' 
>> of the skolemization. By that I mean: what are the 'units' (I do not 
>> know how to call this) within which two 'identical' blank nodes are 
>> skolemized with the same new URI? For OWL:

If I have two statement _somewhere_

SubClass(A _:x)


SubClass(B _:x)

under exactly what circumstances can I be assured that, during 
skolemization, the _:x symbol mapped on the same Skolem constant? Should 
these two statements be in the same ontology? The same file? The same ???.

I am not saying these are _very_ complex questions but they have to be 
specified and are not clear to me.

> I am not sure if I get the question.
>>  - are we speaking about an 'ontology' as being one 'unit'? Or are the 
>> ABox and TBox separated in this sense? I heard different remarks used 
>> on the calls, that is why I ask (I may have misunderstood something).
> In general, axiom is the unit of ontology.  An ABox is a set of 
> individual axioms, while a TBox is a set of class/property axioms. Given 
> an ontology O and its ABox A and its TBox T, O is the union of A and T.

Jeff, I know that. However: if, within the same ontology, I have

SubClass(_:x B)


ClassAssertion( y _:x)

is it so defined that the _:x symbol is mapped on the same skolem 
constant or not? Or are the Abox and Tbox treated separately. Again: I 
may have misunderstood some remark on the call, I just wanted to have a 
clear picture.

>>  - how does this affect the import mechanism? Is skolemization done 
>> after or before all imports? (I would expect 'after', but I just 
>> wanted to be sure...)
> After, so to speak.
>>  - I expect that the 'left' side and the 'right' side of an inference 
>> are skolemized separately (this is what one of Jeremy's test case 
>> says), but I also heard remarks on the call that only the left side is 
>> skolemized and the right side isn't... Or, again, did I misunderstand 
>> something?
> Could you point out which test case from Jeremy that you refer to?



test #5. Bijan's answer


seems to suggest that this is not 100% clear (or not 100% decided...)


> Jeff
> P.S. I assume that the following questions are extensively discussed 
> already.
>> 2. RDFS
>> I would like to understand how exactly the proposed changes affect the 
>> semantic compatibility of OWL-Full and RDFS, ie, where would the 
>> differences be. My current understanding is that the semantics of OWL 
>> Full is a suitable extension of the RDFS semantics, ie, everything 
>> that is true in RDFS is also true in OWL Full. Will that relationship 
>> break? I would expect so, but I would like to have examples on how 
>> this would diverge... Jeff, would it be possible to find an example 
>> where an RDFS inference would no longer be true in a changed OWL1.1 Full?
>> A related question is whether it is possible to change the semantics 
>> of all this on the DL interpretation only. There might be still 
>> backward incompatibilities between OWL1.1 DL and OWL1.0 DL (I guess 
>> your example is one of those) but it might restrict them so that it 
>> would not affect, RDFS+OWL Full usage too much... Would that line make 
>> sense at all?
>> Thanks
>> Ivan
>> Jeff Z. Pan wrote:
>>>> Action 67: Jeff to lead effort on formulating some examples on
>>> b-nodes issues and their impact on users
>>> As Boris pointed out in the telecon, there was already a nice
>>> example (hidden behind some rather technical discussions) in his
>>> earlier email:
>>> http://lists.w3.org/Archives/Public/public-owl-wg/2007Nov/0177.html
>>> In short, there are two choices for semantics of anonymous
>>> individuals (b-nodes):
>>> 1) existentially quantified variables
>>> 2) skolem constants
>>> Example:
>>> Given an ontology O about friends (suppose there are no anonymous
>>> individuals in O). Let us consider the following extra individual
>>> axioms (where :_1 is an anonymous individual):
>>> hasFriend(Bob,:_1)
>>> hasAge(:_1,"26"^^xsd:integer)
>>> With both semantics, the axioms both roughly say "Bob has some
>>> friend aged 26" with some subtle difference: under semantics 1),
>>> the friend aged 26 could be someone already mentioned in O, while
>>> under semantics 2), the friend is someone new and cannot be
>>> someone mentioned in O.
>>> (The above is true unless we have some further extra axioms
>>> forcing :_1 to be the same as some known individuals.)
>>> Comments/Further examples are welcome.
>>> Jeff


Ivan Herman, W3C Semantic Web Activity Lead
Home: http://www.w3.org/People/Ivan/
PGP Key: http://www.ivan-herman.net/pgpkey.html
FOAF: http://www.ivan-herman.net/foaf.rdf

Received on Saturday, 26 January 2008 09:13:32 UTC

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