protected int getIndex(Graph<V, E> graph, E e, V v) {
   Collection<E> commonEdgeSet = new HashSet<E>();
   for (E another : graph.getIncidentEdges(v)) {
     V u = graph.getOpposite(v, another);
     if (u.equals(v)) {
       commonEdgeSet.add(another);
     }
   }
   int count = 0;
   for (E other : commonEdgeSet) {
     if (e.equals(other) == false) {
       edge_index.put(other, count);
       count++;
     }
   }
   edge_index.put(e, count);
   return count;
 }