Could we have some documentation (like, comments in the code -- which this is generally lacking) for why there are 4 stacks?

I believe the report which is linked in the README makes the 4 versions clear.

With regards to comments I would agree, but I do not have the time to write them. In any case, other files in this repository have the same amount of comments, so I don't see why this one should be an exception.

That report was indeed very helpful, sorry for not looking at it before asking. I added a few comments concerning the basic structure of the files, and some remarks based on meditating over the specs a little.

I may have some more questions, but I think that's easier done via email.

Also, I just noticed you forget to add these files to _CoqProject, so they were not actually built (e.g. by the CI). The files are still compatible with current Iris though :)