There is a set that associates every bounded set with a distinguished element of that set. In effect, it {it chooses} an element from every bounded set.
'(Exists (?S) (And (Set ?S) (Forall (?X) (=> (Member ?X ?S) (Double ?X))) (Forall (?X ?Y ?Z) (=> (And (Member (Listof ?X ?Y) ?S) (Member (Listof ?X ?Z) ?S)) (= ?Y ?Z))) (Forall (?U) (=> (And (Bounded ?U) (Not (Empty ?U))) (Exists (?V) (And (Member ?V ?U) (Member (Listof ?U ?V) ?S)))))))