func (b *block) setFormat(format Format) {
	// Set the magic values.
	switch {
	case format.has(formatV7):
		// Do nothing.
	case format.has(FormatGNU):
		copy(b.toGNU().magic(), magicGNU)
		copy(b.toGNU().version(), versionGNU)
	case format.has(formatSTAR):
		copy(b.toSTAR().magic(), magicUSTAR)
		copy(b.toSTAR().version(), versionUSTAR)
		copy(b.toSTAR().trailer(), trailerSTAR)

	if blk.getFormat() != FormatGNU {
		return nil, ErrHeader
	}
	hdr.Format.mayOnlyBe(FormatGNU)

	var p parser
	hdr.Size = p.parseNumeric(blk.toGNU().realSize())
	if p.err != nil {
		return nil, p.err
	}
	s := blk.toGNU().sparse()
	spd := make(sparseDatas, 0, s.maxEntries())
	for {
		for i := 0; i < s.maxEntries(); i++ {
			// This termination condition is identical to GNU and BSD tar.

	var spd sparseDatas
	var spb []byte
	blk := tw.templateV7Plus(hdr, f.formatString, f.formatNumeric)
	if !hdr.AccessTime.IsZero() {
		f.formatNumeric(blk.toGNU().accessTime(), hdr.AccessTime.Unix())
	}
	if !hdr.ChangeTime.IsZero() {
		f.formatNumeric(blk.toGNU().changeTime(), hdr.ChangeTime.Unix())
	}
	// TODO(dsnet): Re-enable this when adding sparse support.
	// See https://golang.org/issue/22735
	/*

		}
		if v, ok := h.PAXRecords[paxKey]; ok && v == formatPAXTime(ts) {
			paxHdrs[paxKey] = v
		}
	}

	// Check basic fields.
	var blk block
	v7 := blk.toV7()
	ustar := blk.toUSTAR()
	gnu := blk.toGNU()
	verifyString(h.Name, len(v7.name()), "Name", paxPath)
	verifyString(h.Linkname, len(v7.linkName()), "Linkname", paxLinkpath)
	verifyString(h.Uname, len(ustar.userName()), "Uname", paxUname)

			copy(sa.isExtended(), "\x80")
		}
		return sps
	}

	makeInput := func(format Format, size string, sps ...string) (out []byte) {
		// 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[:]...)