ThiporKong

def tsort[A](edges: Traversable[(A, A)]): Iterable[A] = {
    @tailrec
    def tsort(toPreds: Map[A, Set[A]], done: Iterable[A]): Iterable[A] = {
        val (noPreds, hasPreds) = toPreds.partition { _._2.isEmpty }
        if (noPreds.isEmpty) {
            if (hasPreds.isEmpty) done else sys.error(hasPreds.toString)
        } else {
            val found = noPreds.map { _._1 }
            tsort(hasPreds.mapValues { _ -- found }, done ++ found)    
        }