logger.go
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)
}
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)
}
// Okf logs a formatted line with an OK prefix.