bearer/opts.go
func NotBefore(enable bool) Option {
	return func(tok *Bearer) error {
		tok.addNotBefore = enable
		return nil
	}
}
func NotBefore(enable bool) Option {
	return func(tok *Bearer) error {
		tok.addNotBefore = enable
		return nil
	}
}
// NotBefore is an option that toggles whether or not the Not Before ("nbf")
// field is generated for the token.