code: func (agg aggregate) Error() string {
  	if len(agg) == 0 {
  		// This should never happen, really.
  		return ""
  	}
  	if len(agg) == 1 {
  		return agg[0].Error()
  	}
  	result := fmt.Sprintf("[%s", agg[0].Error())
  	for i := 1; i < len(agg); i++ {
  		result += fmt.Sprintf(", %s", agg[i].Error())
  	}
  	result += "]"
  	return result
  }
docs: // Error is part of the error interface.