(* ALL DARTS IN AN f-ORBIT ARE DISTINCT: *)
Definition diff_orb(m:fmap)(z:dart):Prop:=
let nr:= Iter_upb_aux m z (fmap_to_set m) in
(diff_int m z 0 (nr - 1))%nat.

Theorem exd_diff_orb:forall(m:fmap)(z:dart),
inv_hmap m -> exd m z ->
diff_orb m z.