proposition: Lemma length_app : forall l1 l2, length (l1 ++ l2) = length l1 + length l2.
proof: Proof. Proof using. intros. unfold length. rew_list~. Qed.
imports: TLC(LibTactics LibLogic LibOperation LibReflect LibProd LibNat LibInt LibOption LibWf) TLC(LibInt)