(* ajout et retrait d'un message dans une file d'attente *)

Definition Post_message (m0 : Message) (b0 : Bag_of_message)
  (s0 s1 : Site) :=
  change_queue (queue Message) b0 s0 s1 (input Message m0 (b0 s0 s1)).Definition Collect_message (b0 : Bag_of_message) (s0 s1 : Site) :=
  change_queue (queue Message) b0 s0 s1 (first_out Message (b0 s0 s1)).

Lemma alt_post_inc : forall s1 : Site, alternate (b0 s0 owner) -> b0 s0 owner = empty Message \/ last Message (b0 s0 owner) = value Message dec -> alternate (Post_message (inc_dec s1) b0 s0 owner s0 owner).