var f formatter
	v7 := tw.blk.toV7()
	v7.typeFlag()[0] = flag
	f.formatString(v7.name(), name)
	f.formatOctal(v7.mode(), 0)
	f.formatOctal(v7.uid(), 0)
	f.formatOctal(v7.gid(), 0)
	f.formatOctal(v7.size(), int64(len(data))) // Must be < 8GiB
	f.formatOctal(v7.modTime(), 0)
	tw.blk.setFormat(format)
	if f.err != nil {
		return f.err // Only occurs if size condition is violated
	}