Section CARD_EFFECT.
(* consequence sur le nombre *)

Lemma diff_collect_card_mess :
forall (m : Message) (s1 s2 s3 s4 : Site) (b : Bag_of_message),
first Message (b s1 s2) <> value Message m \/ s1 <> s3 \/ s2 <> s4 ->
card_mess m (Collect_message b s3 s4 s1 s2) = card_mess m (b s1 s2).