code: func (l *logger) Debugln(vals ...interface{}) {
  	l.mut.Lock()
  	defer l.mut.Unlock()
  	s := fmt.Sprintln(vals...)
  	l.logger.Output(2, "DEBUG: "+s)
  	l.callHandlers(LevelDebug, s)
  }
docs: // Debugln logs a line with a DEBUG prefix.