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Re: ISSUE: OWL-DL compatibility

From: Chris Welty <cawelty@gmail.com>
Date: Tue, 10 Nov 2009 10:53:32 -0500
Message-ID: <4AF98C7C.8000907@gmail.com>
To: Jos de Bruijn <debruijn@inf.unibz.it>
CC: kifer@cs.sunysb.edu, RIF WG Public list <public-rif-wg@w3.org>

I read this a little more carefully.

Basically, the issue is whether to add some correspondence between rif:subclass 
and rdf:subclass and between rif:type and rdf:type *in the OWL compatibility 
section* of SWC.  Such a correspondence is already there for RDF compatibility, 
but Michael noted that it is not "inherited" by the "OWL-DL" (now know as OWL 
Direct Semantics) section.  So, currently in SWC, the OWL-DL compatibility has 
no correspondence between the rather obvious type/subclass relations in the two 
languages.

I agree this is a problem and should be fixed, and option #1 in Michael's 
analysis, copied below (to leave it as is) is unacceptable.

Option #2 is to just add a sentence to the text saying there is no 
correspondence between owl and rif type/subclass.  This is less than satisfactory.

Option #3 is to "fix" it somehow, and there are two variations there, I'll call 
them 3a (just repeat the correspondences from RDFS in OWL-DL) and 3b (do the 
best possible job mapping between owl and rif subclass).

</chair>I prefer option 3a.  I agree with Jos' analysis of option 3b and think 
it is too big a change.<chair>

As chair, I am also willing to accept 2 or 3a as an oversight and bug fix (I 
personally thought the correspondence between type and subclass were "inherited" 
from the RDF correspondence, so 3a would just make it the way I thought it was), 
however 3b seems to me, procedurally, to be much more significant and requires a 
new last call for SWC.

-Chris

Jos de Bruijn wrote:
>> In today's telecon I was asked to reanimate the issue of OWL compatibility,
>> which was discussed 1 month ago.
>>
>> Here is the relevant message:
>>
>> http://lists.w3.org/Archives/Public/public-rif-wg/2009Sep/0017.html
>>
>> The current situation is a bug, IMO. If it isn't a bug then at least that part
>> of the document is very unsatisfactory and obscure. Jos proposed 3 solutions:
>>
>> 1- leave things as they are, assuming that # and ## are not of interest
>>    to users of RIF-OWL DL combinations
>> 2- explain the use of # and ## in the document (this would certainly not
>>    be a substantive change, so we should not run into procedural problems)
>> 3- define the semantics of # and ## in RIF-OWL DL combinations in a
>>    similar fashion as in RIF-RDF combinations: a one-to-one correspondence
>>    between # and OWL class membership statements and implication between ##
>>    and OWL subclassing.
>>
>> The easiest for him would be to do nothing (1), thus leaving things
>> unsatisfactory and obscure. His next choice is (3), which is also my choice and
>> the "right thing to do."  (3) stretches things a little, but it can be argued
>> that it is a simple fix.
> 
> In my earlier e-mail to Michael referred to I did not say what my
> preference is among the mentioned options.  I guess arguments can be
> made for all three options, so in fact I do not have a strong
> preference, but I do have a concern about option (3): implementation
> might be harder.  If, for example, implementation is done through
> embedding in other rules system, like the embedding of RIF-OWL2RL
> combination in the appendix of the document, quite a few rules need to
> be added for the ## construct.
> In particular, for every pair of distinct class names (A,B), we need to
> add the rule:
> 
> Forall ?x (?x[rdf:type -> B] :- And(?x[rdf:type -> A] A##B))
> 
> This means adding a quadratic number of rules.
> 
> Dealing with # is easy: in the mapping of RIF DL-document formulas to
> RIF documents [1] we simply map a#b to tr'(b)(a). Clearly, we would
> restrict b in formulas a#b to constant symbols.
> 
> 
> [1]
> http://www.w3.org/TR/rif-rdf-owl/#Embedding_RIF_DL-document_formulas_into_RIF_BLD
> 
> 
>> Solution (2) is more work. It fixes the obscurity aspect, not the
>> unsatisfactory aspect of the definitions. So, (3) seems like the best way to
>> proceed.
>>
>> Solution (3) still leaves some problems, which are unrelated to the above
>> issues. In the current semantics, subclassing in RIF implies subclassing in
>> OWL/RDF, but not vice versa.
>>
>> In this regard, I would like to point to my follow-up message
>> http://lists.w3.org/Archives/Public/public-rif-wg/2009Sep/0019.html
>> Here I proposed a stronger semantics, which fixes this non-entailment problem.
> 
> Michael proposed the following semantics:
> 
> {(A,B) | A rdfs:subclassOf B and A != B on the RDF side}
>                 = {(A,B) | A##B on the RIF side}
> 
> I feel that this would take us out of Horn, even when considering Simple
> entailment, because implementation would require (classical) negation.
> At least, that is the only way I current see how this could be
> implemented. As we know, classical negation in the body amounts to
> disjunction in the head, so we would end up adding the following rule to
> the embedding of RDF-RDF combinations:
> 
> Forall ?x, ?y (Or(?x##?y ?x=?y) :- ?x[rdfs:subClassOf -> ?y])
> 
> For RIF-OWL DL combinations such a semantics is even more problematic,
> because subclass in OWL DL means subset relation between class
> extensions, so the condition would look something like (X^C is the class
> extension of X):
> 
> {(A,B) | A^C subset B^C and A^C != B^C on the RDF side}
>                 = {(A,B) | A##B on the RIF side}
> 
> (Actually, we will need to apply some tricks here, since A and B are not
> constants on the OWL side, but I guess we can come up with a definition
> that kind-of achieves this semantics)
> 
> A formula implementing the => direction of the condition for a pair of
> class names A,B would look something like (again, negation in the body
> becomes disjunction in the head):
> 
> Forall ?x (
>   Or(A##B
>      And(Forall ?x(?x[rdf:type -> B] :- ?x[rdf:type -> A])
>          Forall ?x(?x[rdf:type -> A] :- ?x[rdf:type -> B])))
>   :-
>   Forall ?x(?x[rdf:type -> B] :- ?x[rdf:type -> A]))
> 
> 
> So, I would not be in favor of extending either the semantics of RDF or
> the semantics of OWL DL combinations with such a condition.
> 
> 
> Best, Jos
>> This would certainly be a substantive change semantically (although not
>> significant textually). If we don't have the energy to do it this time,
>> maybe for RIF 1.1.
>>
>> michael
>>
> 

-- 
Dr. Christopher A. Welty                    IBM Watson Research Center
+1.914.784.7055                             19 Skyline Dr.
cawelty@gmail.com                           Hawthorne, NY 10532
http://www.research.ibm.com/people/w/welty
Received on Tuesday, 10 November 2009 15:54:17 GMT

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