Definition lzero := cons d0 nil.Definition lunorm l :=
  match lnzhead l with
  | nil => lzero
  | d => d
  end.

(* Specifications: *)
Lemma unorm_spec u : to_list (unorm u) = lunorm (to_list u).
Lemma revapp_spec d d' : to_list (revapp d d') = List.rev_append (to_list d) (to_list d').
Lemma rev_spec d : to_list (rev d) = List.rev (to_list d).
Lemma app_spec d d' : to_list (app d d') = Datatypes.app (to_list d) (to_list d').
Lemma unorm_spec u : to_list (unorm u) = lunorm (to_list u).
Lemma revapp_spec d d' : to_list (revapp d d') = List.rev_append (to_list d) (to_list d').
Lemma rev_spec d : to_list (rev d) = List.rev (to_list d).
Lemma app_spec d d' : to_list (app d d') = Datatypes.app (to_list d) (to_list d').