End INVARIANT6.
Section INVARIANT6_bis.

Lemma inc_dec_in_post2 :
forall (b : Bag_of_message) (s0 s1 s2 s3 : Site) (m : Message),
inc_dec_in_queue s1 (Post_message Message m b s2 s3 s0 owner) ->
m <> inc_dec s1 -> inc_dec_in_queue s1 (b s0 owner).