func (agg aggregate) Error() string {
	if len(agg) == 0 {
		// This should never happen, really.
		return ""
	}
	return fmt.Sprintf("[%s]", strings.Join(agg.Strings(), ", "))
}
func (agg aggregate) Error() string {
	if len(agg) == 0 {
		// This should never happen, really.
		return ""
	}
	return fmt.Sprintf("[%s]", strings.Join(agg.Strings(), ", "))
}
// Error is part of the error interface.