code: func (fv FlagValues) Bool(name string, defaultValue bool) (bool, error) {
  	value, found := fv[name]
  	if !found {
  		return defaultValue, nil
  	}
  	switch value {
  	case "", "t", "T", "true", "True", "1":
  		return true, nil
  	case "f", "F", "false", "False", "0":
  		return false, nil
  	}
  	return false, fmt.Errorf("flag %q: cannot parse boolean from %q", name, value)
  }
docs: // Bool returns the boolean version of a flag, if set.