code: func (f GenericFlag) ApplyWithError(set *flag.FlagSet) error {
  	val := f.Value
  	if fileEnvVal, ok := flagFromFileEnv(f.FilePath, f.EnvVar); ok {
  		if err := val.Set(fileEnvVal); err != nil {
  			return fmt.Errorf("could not parse %s as value for flag %s: %s", fileEnvVal, f.Name, err)
  		}
  	}
  
  	eachName(f.Name, func(name string) {
  		set.Var(f.Value, name, f.Usage)
  	})
  
  	return nil
  }
docs: // ApplyWithError takes the flagset and calls Set on the generic flag with the value
  // provided by the user for parsing by the flag