(*========================================================== LISTS AND RINGS (OF DOUBLE-LINKS) ==========================================================*)
(* dart*dart LISTS : *)
Inductive list:Set :=
lam : list | cons : dart->dart->list->list.
Definition emptyl(l:list):Prop:=
match l with
lam => True
| _ => False
end.

Lemma emptyl_dec:forall l:list,
{emptyl l}+{~emptyl l}.