Yep, the point on not many stores being iterable is valid.

I'm not completely clear how making this extend `ReadableStore[OuterK, ReadableStore[InnerK, V]]` would help in avoiding Iterable though. Seems to me like we need some way to know which `K`s in the underlying store constitute `OuterK` and `InnerK` when executing `get(outerK)`.
Or what am I missing here?