Role-Class is a meta-class. Its instances are classes defined to be the set of all Entities playing a particular Role in some Relation.
To the extent that updates may occur which change the particular set of tuples comprising a relation, being an instance of such a class is dynamically determined.
(<=> (Role-Class ?X) (Exists (?R ?N) (And (Relation ?R) (Natural ?N) (Forall (?Z) (<=> (Instance-Of ?Z ?X) (Exists (?Args) (And (List ?Args) (Holds ?R ?Args) (= (Nth ?Args ?N) ?Z))))))))
(<=> (Qua-Entity ?X0) (Exists (?Rc) (And (Role-Class ?Rc) (Instance-Of ?X0 ?Rc))))