(* OK: *)

Definition P_degree_per(m:fmap)(z:dart)(n1 n2:nat):
  Prop:= inv_hmap m -> exd m z -> 0 < n1 -> n2 <= ndN m ->  
   Iter (f m) n2 z = z.

Theorem degree_unicity:forall(m:fmap)(z:dart)(j k:nat), inv_hmap m -> exd m z -> let p := degree m z in j < p -> k < p -> Iter (f m) j z = Iter (f m) k z -> j = k.