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)))))))