code: func (b Files) rev() string {
  	keys := make([]string, 0, len(b))
  	for k := range b {
  		keys = append(keys, k)
  	}
  	sort.Strings(keys)
  	buf := sha256.New()
  	for _, k := range keys {
  		buf.Write([]byte(k))
  		buf.Write([]byte{0})
  		buf.Write([]byte(b[k]))
  		buf.Write([]byte{0})
  	}
  	return hex.EncodeToString(buf.Sum(nil))[:40]
  }
docs: // rev returns fake revision of given config set.