public void visit(Graph<String, String>.Vertex v) {
            if (!done.contains(v.getLabel())
                    || fileInfoMap.get(v.getLabel()) < currTime) {
                circle.add(v.getLabel());
                if (preReq.get(v.getLabel()) != null) {
                    for (String x : preReq.get(v.getLabel())) {
                        Graph<String, String>.Vertex v1 = makeMap.get(x);
                        if (v1 == null) {
                            System.err.println("unknown target");
                            System.exit(1);
                        } else if (circle.contains(x)) {
                            System.err.println("circular dependency detected");
                            System.exit(1);
                        }
                        _graph.add(v, v1, v.getLabel() + " to "
                                + v1.getLabel());
                    }
                }
            }
        }