Module Export FM := Dec.F.
Definition set_of_points := t.
Definition Point := DecPoints.t.
Definition couple(x y :Point) : set_of_points :=
add x (add y empty).
Definition triple(x y t :Point): set_of_points :=
add x (add y (add t empty)).

Lemma test2 : forall A B C,
Equal
(union (singleton C) (couple A B)) (triple A B C).