Trivially, if a feature is introduced at a type in one module, then it is not appropriate to any type in any other module.
a frame for a TFS only needs to allow for the features appropriate to the 4 Prolog terms require one additional unbound variable per TFS (sub)term in order to preserve the intensionality of the logic -unlike Prolog terms, structurally identical TFS substructures are not identical unless explicitly structure-shared.