// Check if we can use USTAR prefix/suffix splitting.
	var namePrefix string
	if prefix, suffix, ok := splitUSTARPath(hdr.Name); ok {
		namePrefix, hdr.Name = prefix, suffix
	}

	// Pack the main header.
	var f formatter
	blk := tw.templateV7Plus(hdr, f.formatString, f.formatOctal)
	f.formatString(blk.toUSTAR().prefix(), namePrefix)
	blk.setFormat(FormatUSTAR)
	if f.err != nil {

	tok := in.Stack.Next()
	if tok != scanner.Ident {
		in.expectText("expected identifier after '#'")
	}
	if !in.enabled() {
		// Can only start including again if we are at #else or #endif but also
		// need to keep track of nested #if[n]defs.
		// We let #line through because it might affect errors.
		switch in.Stack.Text() {
		case "else", "endif", "ifdef", "ifndef", "line":
			// Press on.
		default:
			return false
		}

func Fields(s []byte) [][]byte {
	// First count the fields.
	// This is an exact count if s is ASCII, otherwise it is an approximation.
	n := 0
	wasSpace := 1
	// setBits is used to track which bits are set in the bytes of s.
	setBits := uint8(0)
	for i := 0; i < len(s); i++ {
		r := s[i]
		setBits |= r
		isSpace := int(asciiSpace[r])
		n += wasSpace & ^isSpace
		wasSpace = isSpace
	}

	}

	// Verify the header matches a known format.
	format := tr.blk.getFormat()
	if format == FormatUnknown {
		return nil, nil, ErrHeader
	}

	var p parser
	hdr := new(Header)

	// Unpack the V7 header.
	v7 := tr.blk.toV7()
	hdr.Typeflag = v7.typeFlag()[0]
	hdr.Name = p.parseString(v7.name())
	hdr.Linkname = p.parseString(v7.linkName())
	hdr.Size = p.parseNumeric(v7.size())