code: func (r aggregate) Error() string {
  	if len(r) == 0 {
  		return ""
  	}
  	output := r[0].Error()
  	for i := 1; i < len(r); i++ {
  		output = fmt.Sprintf("%v, %v", output, r[i])
  	}
  	return output
  }
docs: // Error implements the error interface