Rather than loading the `singleton` stdlib and mixing it in and using `NullNotification.instance`, what do you think about using the class as the singleton?  In general, I prefer to avoid loading more parts of stdlib unless it's really needed; given ruby's `require` model, if we require a library it creates an opportunity for an end-user to use that library, forget the `require`, still have passing tests, and release busted code.  Obviously, this isn't an absolute rule (e.g. it would suck to reimplement the option parser lib) but here `singleton` is doing so little for us that I'd rather just treat the class itself as the singleton.