I agree, if the interface is going to be drastically different then you should not try to subclass. To do so would probably involve overriding the parent's methods with dummy placeholders to prevent the user from accessing inherited but unwanted behavior. In this case though, I'd be surprised if it were not possible to expand the interface to allow extensible argument checking via this newrule method he's proposing, without breaking compatibility with the parent module. But if I'm wrong I fall back on my original caveat :^p