Fixpoint RoseS_spec (x y z : bs) :=
  match z with
    | nil => nil
    | _ :: z' =>
      if leb (length z) (length x - length y)
        then false :: RoseS_spec x y z'
        else hd false (skipn (length x - length z) x) :: RoseS_spec x y z'
  end.

Lemma RoseS_cons : forall x y z b1 b2, length z <= length x -> RoseS_spec (b1 :: x) (b2 :: y) z = RoseS_spec x y z.