errWriteHole       = errors.New("archive/tar: write non-NUL byte in sparse hole")
)

type headerError []string

func (he headerError) Error() string {
	const prefix = "archive/tar: cannot encode header"
	var ss []string
	for _, s := range he {
		if s != "" {
			ss = append(ss, s)
		}
	}
	if len(ss) == 0 {
		return prefix
	}
	return fmt.Sprintf("%s: %v", prefix, strings.Join(ss, "; and "))