code: public void addInternalTriggerListener (final ITriggerListener triggerListener)
    {
      ValueEnforcer.notNull (triggerListener, "TriggerListener");
      ValueEnforcer.notEmpty (triggerListener.getName (), "TriggerListener.getName()");
  
      synchronized (m_aInternalTriggerListeners)
      {
        m_aInternalTriggerListeners.put (triggerListener.getName (), triggerListener);
      }
    }
docs: <p>
  Add the given <code>{@link com.helger.quartz.ITriggerListener}</code> to
  the <code>Scheduler</code>'s <i>internal</i> list.
  </p>