def combs[A](n: Int, l: List[A]): Iterator[List[A]] = n match {
    case _ if n < 0 || l.lengthCompare(n) < 0 => Iterator.empty
    case 0 => Iterator(List.empty)
    case n => l.tails.flatMap({
      case Nil => Nil
      case x :: xs => combs(n - 1, xs).map(x :: _)
    })
  }