code: public AbstractStreamParameters track(String track) {
  		if (this.track.length() > 0) {
  			this.track.append(',');
  		}
  		this.track.append(track);
  		return this;
  	}
docs: Add tracking keywords to the filter.
  Does not replace any existing tracking keywords in the filter.
  @param track the keywords to track.
  @return this StreamFilter for building up filter parameters.