var blk block
		copy(blk[:], b.Bytes())
		prefix := string(blk.toUSTAR().prefix())
		prefix, _, _ = strings.Cut(prefix, "\x00") // Truncate at the NUL terminator
		if blk.getFormat() == FormatGNU && len(prefix) > 0 && strings.HasPrefix(name, prefix) {
			t.Errorf("test %d, found prefix in GNU format: %s", i, prefix)
		}

		tr := NewReader(&b)
		hdr, err := tr.Next()

		// Write the initial GNU header.
		var blk block
		gnu := blk.toGNU()
		sparse := gnu.sparse()
		copy(gnu.realSize(), size)
		sps = populateSparseMap(sparse, sps)
		if format != FormatUnknown {
			blk.setFormat(format)
		}
		out = append(out, blk[:]...)

		// Write extended sparse blocks.
		for len(sps) > 0 {
			var blk block
			sps = populateSparseMap(blk.toSparse(), sps)
			out = append(out, blk[:]...)