End ELEMENTS.
Definition elements (i: interv) : list Z :=
elements_rec (fst i) (snd i).

(* Specifications: *)
Lemma in_elements:
forall x i,
In x i -> List.In x (elements i).
Lemma elements_in:
forall x i,
List.In x (elements i) -> In x i.
Lemma in_elements:
forall x i,
In x i -> List.In x (elements i).
Lemma elements_in:
forall x i,
List.In x (elements i) -> In x i.