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)))))