Definition elements (s : t) : list elt := let (l, _) := elements s in l.

Lemma min_elt_3 : forall s : t, min_elt s = None -> Empty s.
Definition max_elt (s : t) : option elt :=
match max_elt s with
| inleft (exist _ x _) => Some x
| inright _ => None
end.