func (h Header) allowedFormats() (format Format, paxHdrs map[string]string, err error) {
	format = FormatUSTAR | FormatPAX | FormatGNU
	paxHdrs = make(map[string]string)

	var whyNoUSTAR, whyNoPAX, whyNoGNU string
	var preferPAX bool // Prefer PAX over USTAR
	verifyString := func(s string, size int, name, paxKey string) {
		// NUL-terminator is optional for path and linkpath.