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 bs2nat_cons_eq : forall b l1 l2, bs2nat l1 = bs2nat l2 -> bs2nat (b :: l1) = bs2nat (b :: l2).