if (state == null) {
                    cycle.addLast(v);
                    List<String> ret = visitCycle(graph, graph.get(v), stateMap, cycle);
                    if (ret != null) {
                        return ret;
                    }
                    cycle.removeLast();
                    stateMap.put(v, DfsState.VISITED);

            DfsState state = stateMap.putIfAbsent(v, DfsState.VISITING);
            if (state == null) {
                cycle.addLast(v.label);
                List<String> ret = visitCycle(v.children, stateMap, cycle);
                if (ret != null) {
                    return ret;
                }
                cycle.removeLast();
                stateMap.put(v, DfsState.VISITED);
            } else if (state == DfsState.VISITING) {

/**
 * Test ComparableVersion.
 *
 */
@SuppressWarnings("unchecked")
class ComparableVersionTest {
    private ComparableVersion newComparable(String version) {
        ComparableVersion ret = new ComparableVersion(version);
        String canonical = ret.getCanonical();
        String parsedCanonical = new ComparableVersion(canonical).getCanonical();

        assertEquals(
                canonical,
                parsedCanonical,

        List<Artifact> trial = getTrail();

        List<String> ret = new ArrayList<>(trial.size());

        for (Artifact artifact : trial) {
            ret.add(artifact.getId());
        }

        return ret;
    }

    private List<Artifact> getTrail() throws OverConstrainedVersionException {
        if (trail == null) {

            DfsState state = stateMap.putIfAbsent(v, DfsState.VISITING);
            if (state == null) {
                cycle.addLast(v.label);
                List<String> ret = visitCycle(v.children, stateMap, cycle);
                if (ret != null) {
                    return ret;
                }
                cycle.removeLast();
                stateMap.put(v, DfsState.VISITED);
            } else if (state == DfsState.VISITING) {