func VersionLabelForChecksum(checksum string) string {
	sepIndex := strings.Index(checksum, "+")
	if sepIndex < 0 {
		return ""
	}
	return checksum[:sepIndex]
}
VersionLabelForChecksum returns the label for the given tarsum
checksum, i.e., everything before the first `+` character in
the string or an empty string if no label separator is found.
package tarsum
import (
	"errors"
	"sort"
	"strconv"
	"strings"
	"github.com/docker/docker/vendor/src/code.google.com/p/go/src/pkg/archive/tar"
)
type Version int
const (
	Version0 Version = iota
	Version1
	VersionDev
)