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 mapi_2 : forall (elt elt':Type)(m: t elt)(x:key) (f:key->elt->elt'), In x (mapi f m) -> In x m.