(* ---------------------------------------------------------------------- *)
(** **** Properties of singulars *)

(* Specifications: *)
(** Environment built from lists. *)
Axiom singles_empty : forall A,
nil ∷ nil = (@empty A).
Axiom singles_cons : forall A x xs (v : A) (vs : list A),
(x :: xs) ∷ (v :: vs) = (xs ∷ vs) & (x ∶ v).
Lemma singles_empty :
nil ∷ nil = (@empty A).
Lemma concat_assoc : forall E F G,
E & (F & G) = (E & F) & G.