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