Admitted. (* QuickChick ev_even_firsttry. (* Existential *) *)
Global Instance testSuchThat_swap_2 {A B : Type} {pre : A -> Prop}
{prop : A -> B -> Type}
`{Checkable (forall a, pre a -> forall b, prop a b)} :
Checkable (forall a b, pre a -> prop a b ) :=
{| checker f := @checker (forall a, pre a -> forall b, prop a b) _ _ |}.
Inductive ev' : nat -> Prop :=
| ev'_0 : ev' 0
| ev'_2 : ev' 2
| ev'_sum : forall n m, ev' n -> ev' m -> ev' (n + m).

Theorem ev'_ev : forall n, ev' n <-> ev n.
Admitted. (* OUT-OF-SCOPE *)