public void setTrace(boolean trace) {
		if ( !trace ) {
			removeParseListener(_tracer);
			_tracer = null;
		}
		else {
			if ( _tracer!=null ) removeParseListener(_tracer);
			else _tracer = new TraceListener();
			addParseListener(_tracer);
		}
	}
During a parse is sometimes useful to listen in on the rule entry and exit
 events as well as token matches. This is for quick and dirty debugging.
runtime/Java/src/org/antlr/v4/runtime/Parser.java