A Set-Class all of whose instances are sets whose members are all of Class Vendor.
(=> (Set-Of-Vendors ?Vendors) (Forall (?X) (=> (Member ?X ?Vendors) (Vendor ?X))))
(<=> (Set-Of-Vendors ?Vendors) (And (Eo-Set ?Vendors) (Set ?Vendors) (Forall (?X) (=> (Member ?X ?Vendors) (Vendor ?X)))))