scratch = DFN--;
scratch = DFN-1;
  public final void markDFN(int DFN) {    setDfsVisited();    for (OPT_SpaceEffGraphEdge e = _outEdgeStart; e != null; e = e.nextOut) {      OPT_SpaceEffGraphNode n = e.toNode();      if (!n.dfsVisited())        n.markDFN(DFN);    }    scratch = DFN--;  }