[sc34wg3] TM Data Model issue: assoc-role-player-type

[Geir Ove Grønmo]

* Kal Ahmed
| I think it makes more sense to require the [role playing topic] property
| to be non-null. 
| 
| If you want to say that  the player is unknown, then you should be
| required to create a topic to represent the "unknown" entity. After all,
| if you say that the role player is unknown, then you are saying that
| there is a subject that is the role player in the association (topic
| maps don't allow fuzzy statements) and it is clear that the one thing
| known about the role playing subject is that it plays a role in an
| association. 

I like this solution: mandatory role types and "blank" topics for
"missing" role player references.

This should cover both the mandatory [type] and [role playing topic]
properties view and the current optional content model of association
roles in XTM.

Cheers,
Geir O.