Order matters for these two collection types, and the collection clearly knows the indexing internally in the _list member (I know, implementation detail, but it would have to know somehow either way), so it is very unfortunate that you can't really treat interface with the indexes in many meaningful ways. To list two instances that I frequently run into this problem:

1. Would of course be nice to just be able to do orderedMap.at(index). This is my main gripe right now, the fact that I need to store out an almost identical separate collection that's just a List to be able to go to index -> item.

2. Would be nice to have index show up in things like map and reduce (filter, etc.). Given that the API for .map is already (item, key, collection), perhaps we could do (item, key, collection, index) to not have any backwards-incompatible changes.