(** Extract the n-th element of a list, as [List.nth_error] does,
    but the index [n] is of type [Z]. *)

Fixpoint list_nth_z (A: Type) (l: list A) (n: Z) {struct l}: option A :=
  match l with
  | nil => None
  | hd :: tl => if zeq n 0 then Some hd else list_nth_z tl (Z.pred n)
  end.

Lemma list_map_exten: forall (A B: Type) (f f': A -> B) (l: list A), (forall x, In x l -> f x = f' x) -> List.map f' l = List.map f l.