> I think this is a bad example.
> #isAbstract used for different purpose, not what we're discussing
> here, i.e. it doesn't tells what kind of protocol an object may
> support, but rather used to test the state of object (in this case
> object is a class being abstract or not).

that's a thin line here. whether something is a protocol or a a query on 
the (static) state of an object is a matter of point of view, internal 
representation of the software, and eventually, taste :)

regards,

Stef