if(g == null) { return; }
	public PGraph(Graph g) {		super(g instanceof DGraph);		_g = g;		Iterator vi = g.getVertices().iterator();		while(vi.hasNext()) {			addNode(new PNode((Vertex) vi.next()));		}		Iterator ei = g.getEdges().iterator();		while(ei.hasNext()) {			Edge e = (Edge) ei.next();			DefaultNode car = PNode.v2p((Vertex) e.getEnds().car());			DefaultNode cdr = PNode.v2p((Vertex) e.getEnds().cdr());			addEdge(new PEdge(e, car, cdr));		}	}