Oh wait, I'm not sure, the API says that forEach will only guarantee calling the callback on each draft element.  It won't call it on the array or the index.  So, your test function below doesn't hold up because my function breaks.  It's not guaranteed to take a callback like that, so that's not a valid test, I think?

Drafts.List.forEach(callback) says it expects the callback to expect a single Draft object. (I'm adding this to the signature comment above)
There is no mutating that can happen that way, right?