(* WE HAVE: z^(i + n*p) = z^i FOR ALL i *)

Lemma Iter_period:forall(m:fmap)(z:dart)(i n:nat),
inv_hmap m -> exd m z ->
let p:= Iter_upb m z in
Iter (f m) (i + n*p)%nat z = Iter (f m) i z.