how to use Set#groupBy in Scala 2.7

groupBy.scala

/*
 * Set#groupBy() is not defined in Scala 2.7.
 */
class GroupableSet[A](val set: Set[A]) {
  def groupBy[K](grouping: A => K): Map[K, Set[A]] = {
    val init: Map[K, Set[A]] = Map()
    set.foldLeft(init) { (map, e) =>
      val key = grouping(e)
      val group = map.getOrElse(key, Set())
      map + (key -> (group + e))
    }
  }
}

implicit def set2GroupableSet[A](set: Set[A]): GroupableSet[A] = new GroupableSet(set)