docs: @Nonnull
    public EChange addListener (@Nonnull final EventListener aListener)
    {
      ValueEnforcer.notNull (aListener, "Listener");
  
      // Small consistency check
      if (!(aListener instanceof ServletContextListener) &&
          !(aListener instanceof HttpSessionListener) &&
          !(aListener instanceof ServletRequestListener))
      {
        LOGGER.warn ("Passed mock listener is none of ServletContextListener, HttpSessionListener or ServletRequestListener and therefore has no effect. The listener class is: " +
                        aListener.getClass ());
      }
  
      return m_aRWLock.writeLocked ( () -> EChange.valueOf (m_aListener.add (aListener)));
    }