I generally like this pattern when end users are meant to subclass (as the definition with `NotImplementedError` makes it clear that they are to define it in their subclass), but in this case I'm in favor of not having this, for a few reasons:

* This is only meant to be subclassed by us, and we've implemented the override in both cases.
* This code will never be executed.
* Every method def adds to the amount of memory the ruby process uses (and the amount of time it takes to boot) -- so while this one method def probably won't be noticable, I prefer not to add method defs that will never be called, as they can add up over time.

What do you think about removing this?