func IsNegativeDuration(in time.Duration) error {
	if in < 0 {
		return fmt.Errorf("invalid duration: %s", in.String())
	}
	return nil
}

func validatePolicyTargetReferences(targetRefs []*type_beta.PolicyTargetReference) (v Validation) {
	for _, r := range targetRefs {
		v = AppendValidation(v, validatePolicyTargetReference(r))
	}
	return
}