code: func (f BoolFlag) ApplyWithError(set *flag.FlagSet) error {
  	val := false
  	if envVal, ok := flagFromFileEnv(f.FilePath, f.EnvVar); ok {
  		if envVal == "" {
  			val = false
  		} else {
  			envValBool, err := strconv.ParseBool(envVal)
  			if err != nil {
  				return fmt.Errorf("could not parse %s as bool value for flag %s: %s", envVal, f.Name, err)
  			}
  			val = envValBool
  		}
  	}
  
  	eachName(f.Name, func(name string) {
  		if f.Destination != nil {
  			set.BoolVar(f.Destination, name, val, f.Usage)
  			return
  		}
  		set.Bool(name, val, f.Usage)
  	})
  
  	return nil
  }
docs: // ApplyWithError populates the flag given the flag set and environment