def breadth[A] (lt: lBT[A]) = {
  def aux(queue: List[lBT[A]]) : Stream[A] = {
    queue match {
      case (Nil) => Stream.empty
      case (LEmpty :: t) => aux (t)
      case (LNode(x, left, right) :: t) => Stream.cons(x, aux(t ::: List(left(), right())))
    }
  }
  aux (List(lt))
}