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 map_1 : forall (m: t elt)(x:key)(e:elt), MapsTo x e m -> MapsTo x (f e) (map f m).