End ALTERNATE.
Section MESS_ALT.
(* consequence sur la structure alternee *)
Variable b0 : Bag_of_Data Message.
Variable s0 : Site.

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 Message (inc_dec s1) b0 s0 owner s0 owner).