b := BoolFlag(true)
		if s == "" {
			// Empty string is treated as valid.
			*bf = b
		} else if b, err = ParseBoolFlag(s); err == nil {
			*bf = b
		}
	}

	return err
}

// FormatBool prints stringified version of boolean.
func FormatBool(b bool) string {
	if b {
		return "on"
	}
	return "off"
}

// ParseBool returns the boolean value represented by the string.