> I have no idea what CLOS does in this situation.
No worries, I was just curious whether you knew whether this was a solved problem in Dylan or CLOS or some other language.

When faced with a diamond diagram, what are the semantics for isinstance(super_instance, cls)?

I worked on it for a little bit and came up with the following:

def super_isinstance(super_inst, cls):
    'Is the cls in the mro somewhere after the current class?'
    mro = super_inst.__self__.__class__.__mro__
    thisclass = super_inst.__thisclass__
    return cls in mro and mro.index(thisclass) < mro.index(cls)