(* EQUIVALENCE OF ORBITS: *)
(* EASY, OK: *)

(* Specifications: *)
Lemma impls_orb:forall(m:fmap)(x y:dart),
inv_hmap m -> exd m x ->
expo m x y ->
impls (Iter_orb m x) (Iter_orb m y).
Lemma eqs_orb:forall(m:fmap)(x y:dart),
inv_hmap m ->
expo m x y ->
eqs (Iter_orb m x) (Iter_orb m y).