)

func main() {
	if len(os.Args) != 2 {
		fatal("usage: callback testname")
	}
	switch os.Args[1] {
	default:
		fatal("unknown test %q", os.Args[1])
	case "Call":
		testCall()
	case "Callback":
		testCallback()
	}
	println("OK")
}

func fatal(f string, args ...any) {
	fmt.Fprintln(os.Stderr, fmt.Sprintf(f, args...))
	os.Exit(1)
}

type GoCallback struct{}

func (p *GoCallback) Run() string {