[OEP] review for n-ary relations draft

Natasha, Alan,

I was not a WG member when the n-ary relations draft was developed and I 
have several comments:

1) The note misses a section on "problems" or "shortcomings" of the 
approach.  There are two main problems with reification:

a] unitended models: Reification introduces a new notion of identity for 
relation tuples, making it possible for the same relation (i.e. tuple) to 
exist multiple times.  For example, it is possible for this model to 
exist:

Christine has_tumor DR1
DR1 diag_value Breast_Tumor
DR1 diag_prob HIGH
Christine has_tumor DR2
DR2 diag_value Breast_Tumor
DR2 diag_value HIGH

In a logic with relations, clearly it would be impossible for there to be 
two of these, creating a difference between the reification approach and 
the relational approach.  Since OWL does not have equality, there is no 
way to express that they are the same. This is especially critical in a 
logic that allows counting (like OWL), because a cardinality constraint 
(other than 0 or 1) would count these two individuals as different, even 
though they are the same. 

b] Reificaiton creates a maintenance problem if you want to have local 
range or cardinality constraints on some role in the reified relation that 
depends on the class of some other role.  You end up having to build a 
lattice of classes to represent all the possible combinations.  For 
example, we might want to say that a person can buy no more than 1 book, 
whereas a company can buy up to 10.  Expressing this constraint requires a 
special subclass of the reified relation class that represents the 
combination of range restrictions.  The example becomes a bit lame because 
of the domain, but there are plenty of good cases for it.

2) I realize the issue of whether to refer to this as reification or not 
was discussed.  I strongly disagree with the conclusion, however.  The 
solution approaches in the note are both forms of reification.  That there 
is an existing notion of reification in RDF doesn't change the fact that 
in the literature, which predates RDF by decades, this is known as 
reification.  It will take one sentence to separate this reification from 
rdf-reification and properly orient people so they will know what it is 
called and can read more about it if they are interested. 

3) Reified relations have an ontological status, that would be useful to 
mention briefly, again for the purposes of orienting the interested reader 
and giving them a good starting place to read more.  It has been argued 
that reified relations actually represent the event that caused the 
relation to come into existence, and taking this perspective actually 
helps make the usage a little more clear.

4) The note misses a bibliography.  Again, I think it is important in 
these notes to let people know that the semantic web did not invent or 
create these problems, and both the problems and solutions have a long 
history.

I am willing to make these changes and generate a new draft.

-Chris

Dr. Christopher A. Welty, Knowledge Structures Group
IBM Watson Research Center, 19 Skyline Dr., Hawthorne, NY  10532     USA   
 
Voice: +1 914.784.7055,  IBM T/L: 863.7055, Fax: +1 914.784.7455
Email: welty@watson.ibm.com, Web: 
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