code: func (ld LogDescription) Info(exp time.Time) (string, string, error) {
  	if ld.TemporalSet == nil {
  		return ld.URI, ld.Key, nil
  	}
  	shard, err := ld.TemporalSet.pick(exp)
  	if err != nil {
  		return "", "", err
  	}
  	return shard.URI, shard.Key, nil
  }
docs: // Info returns the URI and key of the log, either from a plain log description
  // or from the earliest valid shard from a temporal log set