[topicmaps-comment] Re: RDF/Topic Maps: late/lazy reification vs.early/preemptive reification

[Lars Marius Garshol <larsga@garshol.priv.no>]

* Piotr Kaminski
| 
| So far, so good.  However, the clubs are very exclusive, and
| membership needs to be sponsored.  We, of course, wish to record the
| sponsor(s) of each member of each club.  Sponsorship is not a
| property of the club itself, since it has many members each with
| different sponsors.  It is also not a property of the people
| themselves, since each is a member of multiple clubs, probably with
| different sponsors.

I think SRN is right here: the way to model this is to use a ternary
association, with the sponsor playing the third role.
 
| In TM, the situation is far trickier.  People play roles in a
| membership association.  We'd now like to make statements about each
| "playing".  Unfortunately, TM doesn't preemptively reify this
| relationship, and does not provide us with any special mechanism to
| do so. [3]

Your example is kind of artificial, because there are better ways to
solve this problem, but you whether you can reify association roles in
topic maps or not is a question of which model you use.

In PMTM4 there is no t-node that represents each association role
(please correct me if I am wrong). This is in my opinion a serious
weakness of PMTM4, and it's related to the problem of the three-legged
arc.

In the infoset model there is an association role information item
type, and this item type has a [source locators] property, which means
that you can reify any association role item by putting the same
locator item into its [source locators] property and into the [subject
indicators] property of a topic item.

In short, your statement only becomes true if we adopt PMTM4 without
changing it to fix this problem.

--Lars M.