(* EACH dart OF AN orbit IS ITERATED: *)

(* Specifications: *)
Lemma exds_orb_ex :forall(m:fmap)(z t:dart),
inv_hmap m -> exd m z ->
let s:= Iter_orb m z in
let p:= Iter_upb m z in
exds s t ->
{i : nat | (i < p)%nat /\ Iter (f m) i z = t}.