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

Lemma alt_post_elsewhere :
forall (s1 s2 : Site) (m : Message),
s1 <> s0 \/ s2 <> owner ->
alternate (b0 s0 owner) ->
alternate (Post_message Message m b0 s1 s2 s0 owner).