From: Rinke Hoekstra <hoekstra@uva.nl>

Date: Fri, 30 May 2008 10:24:34 +0200

Cc: "Boris Motik" <boris.motik@comlab.ox.ac.uk>, <public-owl-wg@w3.org>

Message-Id: <DE6F402E-FF34-410D-BCFE-76FF4A64FE70@uva.nl>

To: Michael Schneider <schneid@fzi.de>

Date: Fri, 30 May 2008 10:24:34 +0200

Cc: "Boris Motik" <boris.motik@comlab.ox.ac.uk>, <public-owl-wg@w3.org>

Message-Id: <DE6F402E-FF34-410D-BCFE-76FF4A64FE70@uva.nl>

To: Michael Schneider <schneid@fzi.de>

Tracker, this is related to ISSUE-112 -Rinke On 29 mei 2008, at 23:31, Michael Schneider wrote: > Hi Boris! > > Many thanks for this clear explanation. I think I now understand > what your > concerns are. > > To point 2: What I at least can imagine is that adding U to OWL R > Full would > result in a mess. So let's better not have it in this particular > profile. > > To point 1: I was surprised that the encoding of U was so simple (I > myself > only found a complicated encoding based on sub property chains a few > months > ago). And I can see that it is even simpler, because the reflexivity > axiom > (2) is redundant: For each x in owl:Thing we have U(x,ni) by axiom > (1). So > we have U(ni,x) by symmetry (3). And then U(x,x) follows from > transitivity > (4). This means that the top property can already be expressed in > OWL 1. > > Cheers, > Michael > >> -----Original Message----- >> From: Boris Motik [mailto:boris.motik@comlab.ox.ac.uk] >> Sent: Thursday, May 29, 2008 1:52 PM >> To: Michael Schneider >> Cc: public-owl-wg@w3.org >> Subject: RE: Question about problems with top/bottom property >> >> Hello, >> >> Here is the executive summary of this rather long e-mail: >> >> 1. The universal property can already be expressed in OWL 2 in a >> straightforward way. >> 2. This straightforward encoding (suggested in most papers), >> however, is >> quite inefficient in practice. >> 3. It might be possible to come up with a more efficient >> implementation >> technique. This technique, however, would not be completely >> trivial. >> 4. It is currently unclear how any of these techniques would fare in >> practice. >> 5. It is currently unclear that the potential "dangers" outweigh the >> benefits of adding the universal property. >> 6. Therefore, we might want to wait before we add this feature to >> OWL 2. >> >> And now for the details. >> >> = >> = >> = >> ===================================================================== >> ================= >> >> Point 1. >> -------- >> >> You can use existing OWL 2 axioms to encode universal role. The >> following axioms make U universal (ni is a new individual -- that >> is, an individual that does not occur elsewhere in your ontology): >> >> (1) SubClassOf( owl:Thing hasValue( U ni ) ) >> (2) ReflexiveProperty( U ) >> (3) SymmetricProperty( U ) >> (4) TransitiveProperty( U ) >> >> Axiom (1) makes every individual in the interpretation domain >> connected >> through U to ni, and axioms (2), (3), and (4) then ensure >> that you have a connection between all possible individuals. This >> is the >> encoding suggested as an encoding in most papers: one >> simply adds these axioms to an ontology and treats U as an ordinary >> object property. >> >> Point 2. >> -------- >> >> The problem with this encoding is that axioms (1)--(4) connect >> everything with everything. Consider what would happen if you added >> (1)--(4) to an ontology containing a large number of assertions. >> Then, >> your reasoner would have to deal with the extension of U >> which is at least quadratic in the number of individuals occurring in >> the ontology. This is likely to cause problems for indexing >> and memory storage management in general. >> >> >> Point 3. >> -------- >> >> I had a quick chat with Ian, and we noticed that there might be a >> way to >> implement the universal property more efficiently, without >> the explicit maintenance of the extension of U. Here is a very rough >> sketch how this might work. >> >> The only way that U does something from a logical point of view is >> through universals, and in such cases, U is connected to >> everything; hence, you might find a suitable reformulation of >> AllValuesFrom( U CE ) and simulate it though axioms of the form >> SubClassOf( owl:Thing CE ). In fact, the two constructs are "quite >> close" semantically. >> >> The complication here is with the role hierarchy: one would have to >> ensure that the used encoding does not mess up some interaction >> w.r.t. complex role inclusions. >> >> Thus, there is some conceptual work to be done, albeit this work is >> probably not hard and/or interesting from a purely theoretical >> point of view. >> >> >> Point 4. >> -------- >> >> We should be careful when extending OWL 2 with new features that have >> not been extensively tested in practice. >> >> If we don't have the universal property in OWL 2, then it is user's >> fault if he adds the axioms (1)--(4) to an ontology and >> everything suddenly runs slowly. In fact, if a user complains that my >> reasoner is running slowly on his ontology, I can tell him >> "it's your fault because you are using an ontology which is hard". >> >> If we allow for the universal property, then users will use it (even >> though they might not really need it). But then, if my >> implementation technique for an official feature of OWL 2 is flaky >> (and, >> in particular, if this flakiness occurs in even rather >> simple cases), the user has every right to complain. >> >> >> Point 5. >> -------- >> >> On the one hand, I see that the universal property might be >> intuitively >> appealing: it would allow make the language symmetric when >> compared with classes (which have owl:Thing), and it would allow us >> to >> "hang" the property hierarchy off of the universal role. >> >> On the other hand, I don't see what expressivity benefits we gain by >> adding the construct to the language. As I already mentioned, >> AllValuesFrom( U CE ) and SubClassOf( owl:Thing CE ) are "very close" >> semantically. >> >> Thus, the added expressivity of the universal property does not >> seem to >> outweigh the potential risks identified in Point 4. >> >> >> >> Point 6. >> -------- >> >> My preferred course of action would be to let someone demonstrate >> (either by using the simple encoding (1)--(4) or by developing a >> more efficient implementation approach) that adding universal >> property >> does not really cause problems in practice. Assuming this is >> done, adding the feature to the language should not be contentious. >> >> >> Regards, >> >> Boris >> >>> -----Original Message----- >>> From: public-owl-wg-request@w3.org [mailto:public-owl-wg- >> request@w3.org] On Behalf Of Michael >>> Schneider >>> Sent: 29 May 2008 09:32 >>> To: Boris Motik >>> Cc: public-owl-wg@w3.org >>> Subject: Question about problems with top/bottom property >>> >>> Hi Boris! >>> >>> In yesterday's telco you expressed some concerns about the >> introduction of >>> the top/bottom properties into OWL. But I did not understand what >>> the >>> problem was. Can you please elaborate on this topic. >>> >>> Thanks, >>> Michael >> > ----------------------------------------------- Drs. Rinke Hoekstra Email: hoekstra@uva.nl Skype: rinkehoekstra Phone: +31-20-5253499 Fax: +31-20-5253495 Web: http://www.leibnizcenter.org/users/rinke Leibniz Center for Law, Faculty of Law University of Amsterdam, PO Box 1030 1000 BA Amsterdam, The Netherlands -----------------------------------------------Received on Friday, 30 May 2008 08:25:10 UTC

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