SETOF is the set constructor function for KIF. It takes any finite number of arguments and denotes the set of those things.
(Undefined (Arity Setof))
(Instance-Of (Arity Setof) Undefined) (=> (Setof $X $Y) (Simple-Set $Y)) (= ?X (Setof)) (<=> (Empty ?X) (= ?X (Setof)))