code: func (f Int64SliceFlag) ApplyWithError(set *flag.FlagSet) error {
  	if envVal, ok := flagFromFileEnv(f.FilePath, f.EnvVar); ok {
  		newVal := &Int64Slice{}
  		for _, s := range strings.Split(envVal, ",") {
  			s = strings.TrimSpace(s)
  			if err := newVal.Set(s); err != nil {
  				return fmt.Errorf("could not parse %s as int64 slice value for flag %s: %s", envVal, f.Name, err)
  			}
  		}
  		if f.Value == nil {
  			f.Value = newVal
  		} else {
  			*f.Value = *newVal
  		}
  	}
  
  	eachName(f.Name, func(name string) {
  		if f.Value == nil {
  			f.Value = &Int64Slice{}
  		}
  		set.Var(f.Value, name, f.Usage)
  	})
  	return nil
  }
docs: // ApplyWithError populates the flag given the flag set and environment