Instance ArbNatGenCorrect : Correct nat arbitrary.

Lemma arbPair_correctSize
{A B} `{Arbitrary A} `{Arbitrary B} (Sa : nat -> set A)
(Sb : nat -> set B) s:
(semGenSize arbitrary s <--> Sa s) ->
(semGenSize arbitrary s <--> Sb s) ->
(semGenSize arbitrary s <--> setX (Sa s) (Sb s)).