def floyd[V, E, VW, EW](g: MutableGraph[V, E, VW, EW])(implicit ew: Monoid[EW] with Order[EW]) = {
    for (i <- g.vertices; j <- g.vertices; k <- g.vertices;
         ikw <- g.edge(i, k); kjw <- g.edge(k, j)) {
      val ij = g.edge(i, j)
      if (ij.isEmpty || ((ikw |+| kjw) lt ij.get))
        g.setEdge(i, j, Some(ikw |+| kjw))
    }
  }