Definition multl (l:list nat) : nat :=
  fold_right mult 1 l.

(* Specifications: *)
Lemma multl_app l1 l2 : multl (l1++l2) = multl l1 * multl l2.
Lemma multl_plus_distr_l n l : n * plusl l = plusl (map (fun m => n * m) l).