code: func (k cursorKind) cursorKey(pkg, cursor string) string {
  	blob := sha256.Sum224([]byte(pkg + ":" + cursor))
  	return string(k) + ":" + base64.RawStdEncoding.EncodeToString(blob[:])
  }
docs: // cursorKey is a memcache key for items that link cursor to a previous page.