Definition Equivb m m' :=
(forall k, In k m <-> In k m') /\
(forall k e e', MapsTo k e m -> MapsTo k e' m' -> cmp e e' = true).

(* Specifications: *)
Lemma Equivb_elements : forall s s',
Equivb s s' <-> L.Equivb cmp (elements s) (elements s').
Lemma equal_Equivb : forall (s s': t elt), bst s -> bst s' ->
(equal cmp s s' = true <-> Equivb s s').