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Re: Question about problems with top/bottom property

From: Alan Ruttenberg <alanruttenberg@gmail.com>
Date: Sun, 1 Jun 2008 12:20:27 -0400
Message-Id: <19E77CCC-AD6C-4986-922D-9B7A66ED8416@gmail.com>
Cc: "'Michael Schneider'" <schneid@fzi.de>, <public-owl-wg@w3.org>
To: Boris Motik <boris.motik@comlab.ox.ac.uk>

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 16:21:15 GMT

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