// Make subtype to avoid recursive UnmarshalXML().
	type defaultRetention DefaultRetention
	retention := defaultRetention{}

	if err := d.DecodeElement(&retention, &start); err != nil {
		return err
	}

	switch retention.Mode {
	case RetGovernance, RetCompliance:
	default:
		return fmt.Errorf("unknown retention mode %v", retention.Mode)
	}