code: func ioOutputFormatField(field string) func(*LineData, *bytes.Buffer) error {
  	return func(ld *LineData, b *bytes.Buffer) error {
  		v, exist := ld.Fields[field]
  		if !exist {
  			return nil
  		}
  		_, err := b.WriteString(fmt.Sprintf("%v", v))
  		return err
  	}
  }
docs: // Formatting functoin used to implement the %field:XX% code