func arExportData(archive io.ReadSeeker) (io.ReadSeeker, error) {
	if _, err := archive.Seek(0, io.SeekStart); err != nil {
		return nil, err
	}

	var buf [len(armag)]byte
	if _, err := archive.Read(buf[:]); err != nil {
		return nil, err
	}

	switch string(buf[:]) {
	case armag:
		return standardArExportData(archive)
	case armagt:
		return nil, errors.New("unsupported thin archive")
	case armagb:
		return aixBigArExportData(archive)

			}
			return
		}
		Exitf("cannot open file %s: %v", name, err)
	}
	defer f.Close()

	var magbuf [len(ARMAG)]byte
	if _, err := io.ReadFull(f, magbuf[:]); err != nil {
		Exitf("file %s too short", name)
	}

	if string(magbuf[:]) != ARMAG {
		Exitf("%s is not an archive file", name)
	}

	var arhdr ArHdr
	l := nextar(f, f.Offset(), &arhdr)
	if l <= 0 {