code: func (f *int64SliceFlag) Set(val string) error {
  	i, err := strconv.ParseInt(val, 10, 64)
  	if err != nil {
  		return errors.Reason("values must be 64-bit integers").Err()
  	}
  	*f = append(*f, i)
  	return nil
  }
docs: // Set records seeing a flag value.