func (t Transition) Validate() error {
	if !t.set {
		return nil
	}

	if !t.IsDateNull() && t.Days > 0 {
		return errTransitionInvalid
	}

	if t.StorageClass == "" {
		return errXMLNotWellFormed
	}
	return nil
}

// IsDateNull returns true if date field is null
func (t Transition) IsDateNull() bool {
	return t.Date.IsZero()
}

	if (!e.IsDaysNull() || !e.IsDateNull()) && e.DeleteMarker.set {
		return errLifecycleInvalidDeleteMarker
	}

	if !e.DeleteMarker.set && !e.DeleteAll.set && e.IsDaysNull() && e.IsDateNull() {
		return errXMLNotWellFormed
	}

	// Both expiration days and date are specified
	if !e.IsDaysNull() && !e.IsDateNull() {
		return errLifecycleInvalidExpiration
	}