paxHdrs: map[string]string{paxSize: "8589934592"},
		formats: FormatGNU,
	}, {
		header:  &Header{Mode: 07777777},
		formats: FormatUSTAR | FormatPAX | FormatGNU,
	}, {
		header:  &Header{Mode: 07777777 + 1},
		formats: FormatGNU,
	}, {
		header:  &Header{Devmajor: -123},
		formats: FormatGNU,
	}, {
		header:  &Header{Devmajor: 1<<56 - 1},
		formats: FormatGNU,
	}, {

		wantErr: ErrHeader,
	}, {
		input:    makeInput(FormatGNU, "1234", "fewa"),
		wantSize: 01234,
		wantErr:  ErrHeader,
	}, {
		input:    makeInput(FormatGNU, "0031"),
		wantSize: 031,
	}, {
		input:   makeInput(FormatGNU, "80"),
		wantErr: ErrHeader,
	}, {
		input: makeInput(FormatGNU, "1234",
			makeSparseStrings(sparseDatas{{0, 0}, {1, 1}})...),

			}
			if h.Typeflag == TypeGNUSparse {
				whyOnlyGNU = "only GNU supports TypeGNUSparse"
				format.mayOnlyBe(FormatGNU)
			} else {
				whyNoGNU = "GNU supports sparse files only with TypeGNUSparse"
				format.mustNotBe(FormatGNU)
			}
			whyNoUSTAR = "USTAR does not support sparse files"
			format.mustNotBe(FormatUSTAR)
		}
	*/

	// Check desired format.

		return FormatGNU
	default:
		return formatV7
	}
}

// setFormat writes the magic values necessary for specified format
// and then updates the checksum accordingly.
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)

				Size:     16 << 30,
				Mode:     0640,
				Uid:      73025,
				Gid:      5000,
				Uname:    "dsymonds",
				Gname:    "eng",
				ModTime:  time.Unix(1254699560, 0),
				Format:   FormatGNU,
			}, nil},
		},
	}, {
		// This truncated file was produced using this library.
		// It was verified to work with GNU tar 1.27.1 and BSD tar 3.1.2.
		//  dd if=/dev/zero bs=1G count=16 >> writer-big-long.tar

	// Unfortunately, the STAR format also has a sparse header format that uses
	// the same type flag but has a completely different layout.
	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()

	if len(hdr.Name) > nameSize {
		data := hdr.Name + "\x00"
		if err := tw.writeRawFile(longName, data, TypeGNULongName, FormatGNU); err != nil {
			return err
		}
	}
	if len(hdr.Linkname) > nameSize {
		data := hdr.Linkname + "\x00"
		if err := tw.writeRawFile(longName, data, TypeGNULongLink, FormatGNU); err != nil {
			return err
		}
	}

	// Pack the main header.