def choose(n: Int, c: Int): Set[Set[Int]] =
    if (c == 0)
      Set(Set())
    else {
      val rec: Set[Set[Int]] = choose(n, c - 1)
      1.to(n).flatMap(i => rec.map(_ + i).filter(_.size == c)).toSet
    }
  def main(args: Array[String]): Unit = {
    val xs = choose(7, 4)
    val xss = for (x <- xs; y <- xs; z <- xs) yield List(x, y, z)
    println(xss.size)
    val answer = xss.filter(e => (e(0) ++ e(1) ++ e(2)).size ==7)
    println(answer.size)
  }