(* 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) _ _ |}.

Theorem ev_sum : forall n m, ev n -> ev m -> ev (n + m).