(* CHARACTERIZATION OF orbit: *)

Theorem exds_orb_eq_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 <->
exists i:nat,(i < p)%nat /\ Iter (f m) i z = t).