(*===========================================================
                between in f-ORBITS:
==========================================================*)

(* v IS between z AND t IN AN f-ORBIT: *)

Definition between(m:fmap)(z v t:dart):Prop:=
 inv_hmap m -> exd m z ->  
  exists i:nat, exists j:nat, 
   Iter (f m) i z = v /\
     Iter (f m) j z = t /\
       (i <= j < Iter_upb m z)%nat.

Lemma between_expo:forall(m:fmap)(z v t:dart), inv_hmap m -> exd m z -> between m z v t -> expo m z v /\ expo m z t.