code: func (f Int64Flag) ApplyWithError(set *flag.FlagSet) error {
  	if envVal, ok := flagFromFileEnv(f.FilePath, f.EnvVar); ok {
  		envValInt, err := strconv.ParseInt(envVal, 0, 64)
  		if err != nil {
  			return fmt.Errorf("could not parse %s as int value for flag %s: %s", envVal, f.Name, err)
  		}
  
  		f.Value = envValInt
  	}
  
  	eachName(f.Name, func(name string) {
  		if f.Destination != nil {
  			set.Int64Var(f.Destination, name, f.Value, f.Usage)
  			return
  		}
  		set.Int64(name, f.Value, f.Usage)
  	})
  
  	return nil
  }
docs: // ApplyWithError populates the flag given the flag set and environment