(** To make environments easy to read, instead of building them from [nil] and [cons], we prefer to build them from the following components: - [nil]: The empty list. - [one]: Lists consisting of exactly one item. - [++]:  List append.  Furthermore, we introduce compact notation for one (singleton lists): [(x ~ T)] is the same as [one (x, T)]. *)
(** The simpl_env tactic actually puts lists built from only nil, one and [++] into a "normal form". This process reassociates all appends to the right, removes extraneous nils converts cons to singleton lists with an append. *)

Lemma simpl_env_demo : forall (x y:atom) (T1 T2:typ) (E F:env),
((x ~ T1) ++ nil) ++ (y,T2) :: (nil ++ E) ++ F =
(x ~ T1) ++ (y ~ T2) ++ E ++ F.