(** [list_prod] has the same signature as [combine], but unlike
     [combine], it adds every possible pairs, not only those at the
     same position. *)

    Fixpoint list_prod (l:list A) (l':list B) :
      list (A * B) :=
      match l with
      | [] => []
      | x :: t => (map (fun y:B => (x, y)) l')++(list_prod t l')
      end.

Lemma length_prod : forall (l:list A)(l':list B), length (list_prod l l') = (length l) * (length l').