(* We are now ready to define our multiplication *)
Fixpoint join (n : nat) : vect n -> vect n -> vect n :=
match n return (vect n -> vect n -> vect n) with
| 0%nat => fun x y => (x * y)%f
| S n1 =>
fun x y =>
let (x1, x2) := x in
let (y1, y2) := y in (join n1 x1 y2 + join n1 (x2 ^_'f) y1,
join n1 x2 y2)
end.
(* Unicode u2228 *)
Notation "x '∨' y" := (join _ x y) (at level 40, left associativity): type_scope.

(* x*x = 0 is stable by linear combination *)
Lemma cbl_join_id n vs :
(forall x y, In x vs -> In y vs -> join n y x = (- (1)).* (x ∨ y)) ->
(forall x, In x vs -> x ∨ x = 0) -> forall x, cbl _ vs x -> x ∨ x = 0.