proposition: Lemma wp''_concat : forall l1 l2:list par, wp'' l1 -> wp'' l2 -> wp'' (l1 ++ l2).
proof: Proof. intros l1 l2 H1 H2; generalize l1 H1; clear H1 l1; elim H2. - intros; rewrite app_nil_r; trivial. - intros; rewrite app_assoc; auto. Qed.
imports: List Arith