@rud The idea here is that a `Feature` is only created internally, so the `opts[:use_as_id]` is always populated using the `user_identifier_method` method, which provides that fallback. Moreover, the `@use_as_id` variable is private.

So, I think the only way of this going wrong is the user creating a `Feature` instance and not passing it the `:use_as_id`. Is this a real problem? I can add a fallback if it is, no problem.