Instance ArbNatGenCorrect : Correct nat arbitrary.

Lemma arbList_correct:
forall {A} `{H : Arbitrary A} (P : nat -> A -> Prop) s,
(semGenSize arbitrary s <--> P s) ->
(semGenSize arbitrary s <-->
(fun (l : list A) => length l <= s /\ (forall x, List.In x l -> P s x))).