I agree with bernard, it is not clear in the RM how I model
symmetrical relationships. Such as his sibling example.

* Sam Hunting
|
| (2) Is the best/only way to model the prose statement "Hot and cold
| are opposites" an assertion with a single role player that
| represents the set {"hot","cold"}
That seems quite a funny way of doing it to me, and structurally
different from how topic maps represent this, which would more
naturally be
opposites(hot : opposite, cold : opposite)
In the SAM this is an association item with two role items in its
[roles] property. The transformation to the RM is more involved than
people will expect it to be, and it does seem a little odd.
I realize the rationale is that there should be a node that allows us
to speak about both at the same time, but I'm not sure what the
argument that leads from there to the construction of the set is.

> Why, though, is it necessary to define two different
> role types, even though one would do the job?
>
> The answer is that without the rules:
>
>  (1) that all role types must be different, and
This, I feel, is the role that is in error in the current RM.
>  (2) that there can be only one role player per role type,
This is the role that is actually needed.
> ...the Subject Location Uniqueness Objective can't be
> supported.
I cannot agree with this statement. SLUO can be supported more simply by
requiring the pair to be unique.