Lemma not_in_queue_post : forall (b : Bag_of_message) (s1 s2 s3 s4 : Site) (m1 m2 : Message), m1 <> m2 -> ~ In_queue Message m1 (b s1 s2) -> ~ In_queue Message m1 (Post_message Message m2 b s3 s4 s1 s2).