Definition exists_ (f : elt -> bool) (s : t) : bool :=
if exists_ (P:=fun x => f x = true) (f_dec f) s
then true
else false.

Lemma exists_1 :
forall (s : t) (f : elt -> bool),
compat_bool E.eq f -> Exists (fun x => f x = true) s -> exists_ f s = true.