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

From: Ivan Herman <ivan@w3.org>
Subject: Re: Action-67 some examples on b-nodes issues and their impact on users
Date: Sat, 26 Jan 2008 10:13:28 +0100

> 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)
> 
> and
> 
> 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)
> 
> and
> 
> 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.

Although there is as of yet no complete proposal on the table, I would
assume that the answer here would be exactly the same as for the question
	What is the scope of bnode IDs?
i.e., if it is the same *bnode*, then it gets the same skolem.

> >>  - 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?
> 
> See
> 
> http://www.w3.org/mid/478DF5F9.8050400@hpl.hp.com
> 
> test #5. Bijan's answer
> 
> http://www.w3.org/mid/C82B011B-E528-4AA7-8505-25172C9C2143@cs.man.ac.uk
> 
> seems to suggest that this is not 100% clear (or not 100% decided...)

Again, what is not clear is the identity of bnodes.  If that is cleared
up, then the skolemization answers are clear.

> Ivan

peter

Received on Saturday, 26 January 2008 13:06:43 UTC