def permutations[T]: List[T] => Traversable[List[T]] = {
    case Nil => List(Nil)
    case xs => {
      for {
        (x, i) <- xs.zipWithIndex
        ys <- permutations(xs.take(i) ++ xs.drop(1 + i))
      } yield {
        x :: ys
      }
    }
  }