def mapUnfold[B](f: A => B): Stream[B] =
    Stream.unfold(unCons) {
      case Some((h, t)) => Some(f(h), t.unCons)
      case _ => None
    }

  def takeUnfold(n: Int): Stream[A] =
    Stream.unfold((unCons, n)) {
      case (Some((h, t)), n) if n > 0 => Some(h, (t.unCons, n - 1))
      case _ => None
    }

  def takeWhileUnfold(p: A => Boolean): Stream[A] =
    Stream.unfold(unCons) {
      case Some((h, t)) if p(h) => Some(h, t.unCons)
      case _ => None
    }
  def zipUnfold[B](that: Stream[B]): Stream[(A, B)] =
    Stream.unfold((unCons, that.unCons)) {
      case (Some((x, xs)), Some((y, ys))) => Some((x, y), (xs.unCons, ys.unCons))
      case _ => None
    }

  def zipAllUnfold[AA >: A, B](that: Stream[B], thisElem: AA, thatElem: B): Stream[(AA, B)] =
    Stream.unfold(unCons, that.unCons) {
      case (Some((x, xs)), Some((y, ys))) => Some((x, y), (xs.unCons, ys.unCons))
      case (Some((x, xs)), None) => Some((x, thatElem), (xs.unCons, None))
      case (None, Some((y, ys))) => Some((thisElem, y), (None, ys.unCons))
      case _ => None
    }