Re: [OEP] Draft of a note on n-ary relations

>People seem to have agreed that doing a pattern on n-ary (reified) 
>relations would be a useful thing to have. Alan Rector and I 
>actually had a chance to work it out and you can see the first draft 
>of our effort at
>http://lists.w3.org/Archives/Public/www-archive/2004May/att-0003/n-aryRelations.html
>
>It's nowhere near as complicated or as controversial (we hope) as 
>the Classes as Values one. In fact, it's rather simple, almost too 
>simple to be a pattern. On the other hand, it seems to be on a topic 
>that many newcomers to OWL have questions on.
>
>As usual, please feel free to poke holes in it and all feedback is welcome.

Several comments.

1. As others have noted, would be best to avoid the word 'reify' here 
as it become poisoned (and in fact it already was, as we discovered a 
while ago: it has many 'tight' meanings in different communities, RDF 
being now just one more of them.) In any case its not really needed, 
one can use an anglo-saxon phrasing and say something like
....such as these is to make the relation into an object...."

2. Strictly, this way of describing it is wrong: the thing that gets, 
er, reified is not the relation, but an instance of the relation (A 
tuple rather than set of tuples: sometimes called a trope or a 
situation or a holding, or more prosaically a fact or even an event). 
If you aren't clear about this the account gets kind of murky: for 
example, in your example, the relation is a 3-argument thing, but the 
orange node has role-style links only to two arguments: so if it is a 
relation is is binary or trinary? But if it was binary, why did you 
need to do anything about it in the first place? And what is the 
relationship between the relation Diagnosis_Relation and the property 
link labelled has_diagnosis? Shouldnt there be some kind of actual 
(not reified) property to which the reified property is somehow 
related? How?? And so on.

I'd suggest that it would  be better to say something like his:

"If we need to...., we can do it by creating individuals which stand 
for an instance of the relation, and relate them to the things that 
are involved in that instance of the relation. The original relation 
can be thought of then as the class of all these relation-instances. 
This is sometimes described as a 'role-value' style of description, 
where the various arguments are seen as playing a 'role' in the fact 
or event indicated by the instance of the relation."

3. Most importantly, I don't like the note saying that this is 'THE 
general solution' . It is one of several options, and not clearly 
always the best one. OWL for example frequently uses a different 
option, where the second argument of the relation is made into a list 
of the remaining arguments: this has many practical advantages, the 
most important being that the property stays a genuine property. 
Other options still are a systematic role-value style of expressing 
things (which your example could be, in fact: its not quite clear) 
and other 'container' style solutions, and its even possible, though 
clunky, to use things like binary subProperties to play the role of 
the component links.

4. I think that the discussion of the two cases distinguished by 
whether there is an 'originator' is best omitted. It is potentially 
misleading and has no connection whatever with the OWL semantics, and 
in any case isnt needed for the point being made.

Pat


>Thanks in advance,
>
>Natasha and Alan


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Received on Friday, 7 May 2004 14:46:29 UTC