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Re: A use case for anon nodes - action from telecon

From: Graham Klyne <Graham.Klyne@Baltimore.com>
Date: Thu, 19 Jul 2001 18:53:43 +0100
Message-Id: <>
To: Brian McBride <bwm@hplb.hpl.hp.com>
Cc: rdf core <w3c-rdfcore-wg@w3.org>
At 11:27 AM 7/19/01 +0100, Brian McBride wrote:
>Loosely in English it means advert123 is for a service that will
>buy roses in quantities of at least 100.
>                      advert123 role buyer
>and  thereExists ?X  advert123 description ?X
>                      ?X        product      roses
>      thereExists ?Y  ?X        minQuantitiy ?Y
>                      ?Y        units        kg
>                      ?Y        minValue     100
> >
> > There seems to me to be no way of rendering this statement using just
> > existential quantification.
>As  you see, I've made an attempt.

Good, thanks!

The problem I now see is that this asserts the existence of the required 

   thereExists ?X which is the object of (advert123 description ?X)

But the apparent intent of this is ask if such a service exists.  Do I 
detect a "gensym" error?

> >
> > This may be a compelling use-case, but I don't see any sanction for this
> > usage in M&S 1.0, and as such would suggest it be deferred to V2.0.
>What is the difference between this and the example in:
>   http://lists.w3.org/Archives/Public/www-archive/2001Jun/att-0021/00-part#41

That case asserts the existence of the un-identified individual.

> > >And here is a supplier who can offer a range of services:
> > >
> > >#advert456 :role "seller";
> > >            :description [:product :roses;
> > >                          :quantity [:units :kg; :maxValue "500"]].
> >
> > I think this case can be expressed adequately using just existential
> > quantification
> >
> >     There exists an X such that:
> >       X is for sale AND
> >       There exists a Y such that:
> >         X :description Y AND
> >         Y :product :roses AND
> >         There exists a Z such that:
> >           Y :quantity Z AND
> >           Z :units :kg AND
> >           Z :maxValue "500".
>Can you account for the fact that both adverts are very similar in
>structure, yet you assign them different semantics?

Well, the difference I perceive is the apparent intent in one case to make 
a statement about something that may or may not exist, and in the other 
case to assert that something does exist.

> > In this case, I think the meaning can be conveyed using either of the
> > approaches we have discussed on the list and in the last teleconference 
> [1].
>This is the essential point.  My colleagues believe that if a resource
>is not anonymous they will process it differently - i.e. it means
>something different.

See below.

> >
> > >Now. If we don't have anonymous nodes then we have the following problems.
> > >
> > >(1) In the seller advert it would appear that the seller is only 
> advertising a
> > >single specific (but under-specified) service, #anon12345 or whatever, 
> which
> > >would be hard to distinguish from an actual service instance like 
> #service42.
> >
> > I would refer to Pat's explanation, copied in [1].  Skolemization seems to
> > work just fine here.
>I have reread that message and it has not helped me to understand.
>The issue is really very simple.  If an anonymous node is used, it means
>"a service selling roses in quantities of at least 100".  If a node with a 
>is given it means "The service called U selling roses in quantities of least
>100".  The difference is that a processor of the advert is expected to 
>know how
>to relate U to the service it denotes.  At least that's how my colleagues
>are using it.

In the seller case, I thought Pat's note explained quite well how a skolem 
form or existential quantification gave you the same information (excerpt 
repeated below).  Maybe much hinges on "If all you know about 'genid:xyzzy' 
is some sentence that I send you"?

If one happens to know something else about "The service called U", such as 
"how to relate U to the service it denotes", then this is a different 
situation.  But I don't think it's brought about by the anonymity, but by 
the expected other knowledge.


At 02:42 PM 7/5/01 -0700, pat hayes wrote:
 > If all you know
 >about 'genid:xyzzy' is some sentence that I send you, then all you really
 >know is (exists (?x)(<my sentence with' ?x' instead of 'genid:xyzzy'>)),
 >and in fact that is all you can infer, apart from such dumb-ass things as
 >(= genid:xyzzy genid:xyzzy). Now of course you are free to generate
 >another genid of your own, and use that as a skolem form; but then you
 >could do that anyway, if I were to give you the existential. There really
 >isnt any inferential advantage to having the existential form over the
 >skolem form.

Graham Klyne                    Baltimore Technologies
Strategic Research              Content Security Group
<Graham.Klyne@Baltimore.com>    <http://www.mimesweeper.com>
Received on Thursday, 19 July 2001 15:52:33 UTC

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