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RE: Question about problems with top/bottom property

From: Boris Motik <boris.motik@comlab.ox.ac.uk>
Date: Sun, 1 Jun 2008 17:36:04 +0100
To: "'Alan Ruttenberg'" <alanruttenberg@gmail.com>
Cc: "'Michael Schneider'" <schneid@fzi.de>, <public-owl-wg@w3.org>
Message-ID: <000101c8c405$9da22930$0200a8c0@wolf>

Hello,

In reasoning services we mention only ontology implication; this includes role problems. This is how things were done in OWL 1.

I guess there is no problem in including the bottom role. As for the top role, we might actually include it eventually; it would
just be good if someone tested it in practice first.

Regards,

	Boris

> -----Original Message-----
> From: Alan Ruttenberg [mailto:alanruttenberg@gmail.com]
> Sent: 01 June 2008 17:20
> To: Boris Motik
> Cc: 'Michael Schneider'; public-owl-wg@w3.org
> Subject: Re: Question about problems with top/bottom property
> 
> Is there any reason not to include bottom role? There is a debugging
> benefit to computing equivalentProperty to bottom role.
> 
> Also, where we discuss computational properties and reasoning
> services - should role subsumption (and with bottom role, role
> satisfiability) be mentioned?
> 
> 
> -Alan
> 
> On May 29, 2008, at 7:52 AM, Boris Motik wrote:
> 
> >
> > 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
> >
> >
> >
Received on Sunday, 1 June 2008 16:37:49 GMT

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