(** [(list_power x y)] is [y^x], or the set of sequences of elts of [y]
      indexed by elts of [x], sorted in lexicographic order. *)

  Fixpoint list_power (A B:Type)(l:list A) (l':list B) :
    list (list (A * B)) :=
    match l with
      | [] => [[]]
      | x :: l =>
        flat_map (fun f:list (A * B) => map (fun y:B => (x, y) :: f) l')
        (list_power l l')
    end.(** find whether a boolean function can be satisfied by an
       elements of the list. *)

    Fixpoint existsb (l:list A) : bool :=
      match l with
        | [] => false
        | a :: l => f a || existsb l
      end.

Lemma existsb_nth : forall l n d, n < length l -> existsb l = false -> f (nth n l d) = false.