(* Iterated contraction *)
Definition mcontra n (ll: list (kn n)) (x: vect n) :=
fold_left (fun x l => #<l,x>#) ll x.
Notation "#<< l , x >>#" := (mcontra _ l x).

Lemma mcontra_swap n a b l (M: vect n) :
#<< a::b::l, M>># = (-(1)).*  #<<b::a::l, M>>#.