proposition: Lemma count_app : forall P l1 l2, count P (l1++l2) = count P l1 + count P l2.
proof: Proof. Proof using. intros. unfold count. rew_listx~. Qed.
imports: Coq.Classes.Morphisms TLC(LibTactics LibLogic LibReflect LibOperation LibProd LibOption LibNat LibInt LibWf LibMonoid LibRelation)