code: func (l *logger) Warnf(format string, vals ...interface{}) {
  	s := fmt.Sprintf(format, vals...)
  	l.mut.Lock()
  	defer l.mut.Unlock()
  	l.logger.Output(2, "WARNING: "+s)
  	l.callHandlers(LevelWarn, s)
  }
docs: // Warnf logs a formatted line with a WARNING prefix.