powerset.scala

object Application extends App {
  def power[A](s: Set[A]): Set[Set[A]] = {
    @annotation.tailrec
    def go(a: Set[A], b: Set[Set[A]]): Set[Set[A]] = {
      if (a.isEmpty) b
      else go(a.tail, b.map(_ + a.head) ++ b)
    }
    go(s, Set(Set()))
  }
  val s = Set("帰", "る")
  val ps = power(s).filter(_ => util.Random.nextBoolean)
  println(ps)
  if (ps.size < 4) println("帰る冪でない")
  else println("帰る冪である")
}