code: private IntervalListener getListener(String intervalName){
  		IntervalListener listener = listeners.get(intervalName);
  		if (listener!=null)
  			return listener;
  
  		try {
  			Interval interval = IntervalRegistry.getInstance().getIntervalOnlyIfExisting(intervalName);
  			listener = new IntervalListener();
  			IntervalListener old = listeners.putIfAbsent(intervalName, listener);
  			if (old!=null)
  				return old;
  			interval.addSecondaryIntervalListener(listener);
  			return listener;
  		}catch(UnknownIntervalException e){
  			return NoIntervalListener.INSTANCE;
  		}
  	}
docs: Returns interval listener for the specified interval. If there already was a listener, it will be returned, otherwise a new one will be created.
  However, if there have been no such interval, the interval listener will be substituted for a dummy object and no further action will be taken.
  @param intervalName
  @return