public void removeLabel(Label label) {
    if (LOG.isDebugEnabled()) {
      LOG.debug("removeLabel(label " + label + ")");
    }

    if (label == null) {
      throw new IllegalArgumentException("Illegal topic argument passed!");
    }

    label.setDiscussionTopic(null);
    labels.remove(label);
  }
  public void addLabel(Label label) {
    if (LOG.isDebugEnabled()) {
      LOG.debug("addLabel(label " + label + ")");
    }

    if (label == null) {
      throw new IllegalArgumentException("topic == null");
    }

    label.setDiscussionTopic(this);
    labels.add(label);
  }