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