code: @Override
  	public void declareOutputFields(OutputFieldsDeclarer declarer) {
  		List<String> names = new ArrayList<>();
  		for (String f : getOutputFields()) names.add(f);
  		for (String f : passThroughFields) names.add(f);
  		String streamId = getOutputStreamId();
  		logger.info("{} declares {} for stream '{}'",
  				new Object[] {this, names, streamId});
  		declarer.declareStream(streamId, new Fields(names));
  	}
docs: Registers the {@link #setPassThroughFields(String...) pass through}
  and the {@link #getOutputFields() output field names}.