rootCandidate.setSignet(this.getSignet());
  public Set getRoots()  {    Set roots = new HashSet();    Iterator nodesIterator = nodes.iterator();    while (nodesIterator.hasNext())    {      TreeNodeImpl rootCandidate = (TreeNodeImpl) (nodesIterator.next());      rootCandidate.setSignet(this.getSignet());      if (rootCandidate.getParents().size() == 0)      {        roots.add(rootCandidate);      }    }    return roots;  }