(* How to generate the multiple product for a constant *)
Fixpoint lgenk (n: nat) (k: K p) {struct n} : list (K p) :=
  match n  with
    O => nil
  | 1 => k :: nil
  | S n1 => k::lgenk n1 k
  end.

Lemma addE_swap x1 x2 x3 : x1 + (x2 + x3) = x2 + (x1 + x3).