Section Properties.
Implicit Types x y : TVar.
Implicit Types xs ys : list TVar.
(* ---------------------------------------------------------------------- *)
(** *** Primary properties *)
(* ---------------------------------------------------------------------- *)
(** **** Properties of singulars *)
(* ---------------------------------------------------------------------- *)
(** **** Properties of concatenation *)

(** Concatenation admits empty as neutral element, and is associative. *)
Axiom concat_empty_r : forall A (E : gen_env A),
E & (@empty A) = E.