public static void RemoveEdge(this Vertex v, Edge e)
 {
     if (v.fromEdges.Contains(e)) {
     Undo.RecordObject(v, "Remove From Edge");
     v.fromEdges.Remove(e);
       }
 }
 public static void AddEdge(this Vertex v, Edge e)
 {
     if (! v.fromEdges.Contains(e)) {
     Undo.RecordObject(v, "Add From Edge");
     v.fromEdges.Add(e);
       }
 }