Nick,
  I've been meaning to reply to you about inverse user relations.
That's pretty high on my agenda, since lots of people seem to want them.

  The main difficulty with implementing it is that it requires implementing
add-slot.  Dynamically adding a slot to a class won't be that hard, although
it will have to be propagted to instances and subclasses.  However, adding
a slot to an instance violates the current constraint that instances have
exactly the same set of slots that their classes have.

  I'll implement inverse relations for classes first, while I (and hopefully
others) think about the implications of doing it for instances, and whether
users need it for instances.  So, what do you all think?

--Peter
