(* OUT-OF-SCOPE *)

(* existential dummy *)
Definition double_reverse n : nat := n.Axiom double_reverse_correct : forall n, n = double (double_reverse n).Instance dec_ex_double n : Dec (exists k, n = double k).

(* Specifications: *)
Lemma ev_even_firsttry : forall n, ev n -> exists k, n = double k.