(** *** Decisions for lists *)
Instance list_in_dec X (x : X) (A : list X) :
eq_dec X -> dec (x el A).
(* Certifying find *)

Lemma cfind X A (p: X -> Prop) (p_dec: forall x, dec (p x)) :
{x | x el A /\ p x} + {forall x, x el A -> ~ p x}.