User model: ref as default, vs universal generics

[Brian Goetz]

Just to record a constraint: there's somewhat of a conflict between the 
idea of "make ref the default", as Kevin advocated, and universal 
generics, which we need to keep in mind as we stack the whole tower.

If a B3 class gives us Foo and Foo.val, then Map::get (currently) has no 
way to declare its return value as "ref T". The plan of record has been:

     V.ref get(K key)

but if V.ref is not denotable, we have a problem.  That means we can't 
*just* have Foo and Foo.val; we need at least to be able to say T.ref 
for type variables, if not Foo.ref for all B3 classes.

If we can manage to use T!, then this is an obvious application for T?, 
but this approach brings new questions.