Definition Equal m m' := forall y, find y m = find y m'.Definition Equiv (eq_elt:elt->elt->Prop) m m' :=
   (forall k, In k m <-> In k m') /\
   (forall k e e', MapsTo k e m -> MapsTo k e' m' -> eq_elt e e').Definition Equivb cmp := Equiv (Cmp cmp).

Lemma equal_2 : forall m m' cmp, equal cmp m m' = true -> Equivb cmp m m'.