This means Cortona is getting it wrong.
No, it's not. You've read the wrong part of the spec. The right one is ``4.8.3 PROTO definition semantics'', which clearly says that ``An exposedField in the prototype interface may be associated only with an exposedField in the prototype definition...".
woops again, yes its only the other way round, a field in the interface can point to an exposedfield somewhere in the definition.