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).

Lemma Equivb_elements : forall s s', Equivb s s' <-> L.Equivb cmp (elements s) (elements s').