code: func formatClusteringKeys(keys []*ClusteringKey) string {
  	pieces := make([]string, len(keys))
  	for index, ck := range keys {
  		pieces[index] = ck.String()
  	}
  	return strings.Join(pieces, ", ")
  }
docs: // formatClusteringKeys takes an array of ClusteringKeys and returns
  // a string that shows all of them, separated by commas