Fixpoint filter_seq (k: cont) : cont :=
 match k with
  | Kseq s :: k1 => filter_seq k1
  | _ => k
  end.

(* Specifications: *)
Lemma cons_app: forall A (x: A) (y: list A), x::y = (x::nil)++y.
Lemma cons_app': forall A (x:A) y z, x::y++z = (x::y)++z.
Lemma cat_prefix_empty: forall {A} prefix (ctl: list A), ctl = prefix ++ ctl -> prefix = nil.