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.

		if err != nil {
			log.Fatalf("loading imports: %v\n%s", err, out)
		}

		var stdPackages []string
		importMap := make(map[string]map[string]string)
		importDir := make(map[string]string)
		dec := json.NewDecoder(bytes.NewReader(out))
		for {
			var pkg struct {
				ImportPath, Dir string
				ImportMap       map[string]string
				Standard        bool
			}
			err := dec.Decode(&pkg)
			if err == io.EOF {
				break