LayerEdge InEdgeOfVirtualNode(int v)
 {
     return(this.BT ? graph.InEdgeOfVirtualNode(v) : graph.OutEdgeOfVirtualNode(v));
 }
 //here u is a virtual vertex
 private LayerEdge OutcomingEdge(int u)
 {
     return(layeredGraph.OutEdgeOfVirtualNode(u));
 }