> I'll change the deprecated methods to `NSValueTransformer *` but any kind of run-time inspection will reveal that they support the protocol (though they won't return meaningful errors).
>
> The adapter will later use run-time inspection, too, but the transformers returned by the deprecated methods should just continue to work as expected. That is, potentially return `nil` but always succeed.

I meant we should return an _actually_ different type (i.e., a different subclass), so it's clear that there's no error handling machinery. I don't think we can assume that `nil` is always success.