ONE-OF is a function for defining classes by enumerating their instances. It takes a variable number of terms as arguments, and denotes the class whose instances are exactly those terms.
For example, (one-of yes no) denotes the class containing the objects called yes or no. (instance-of yes (One-of yes no)) is true, and (instance-of maybe (one-of yes no) is false. A common use for one-of is in defining type restrictions. For example, the relation value-type takes a class as an argument. To specify a finite set of possible values for a slot, one can use (value-type ?instance ?slot (one-of a b c)).
(Undefined (Arity One-Of)) (<=> (= (One-Of @Members) ?Class) (Forall (?Instance) (<=> (Instance-Of ?Instance ?Class) (Member ?Instance (Setof @Members)))))