This is stuff I did as a master student to help me understand set theory and Coq at the same time. This was around 2012-2013.

Given the timing it's probably compatible with Coq 8.4 or maybe 8.3, I don't have either at hand to test.

I started from someone else's development and played with it as I liked. I didn't write down where I got it from, but since the shortened tactics (ir for intros, ap for apply, etc) and some lemma names survived my changes we can tell it was probably https://github.com/coq-contribs/cats-in-zfc (original author Carlos Simpson, see also paper https://arxiv.org/abs/math/0410224 and https://math.unice.fr/~carlos/old/indexFev07.html which has history from before the svn to git import). LICENSE is accordingly LGPL2.1.

Some differences:

• cats-in-zfc has a mix of axiomatic ZFC and using Coq types as sets, i.e. they have Definition E := Type. and then some extra axioms on E. I have purely axiomatic E with Coq serving as the metatheory.

• cats-in-zfc have some stuff in their notation.v to embed methods in their sets or something. I didn't understand it back then (and remember even less now) so I removed it and refactored or removed its users. (the previous point may not have been possible without this change by the way)

• as indicated by the name cats-in-zfc is about categories. I didn't know or care about categories so I deleted those files. Instead I have some trivialities about numbers in set theory.

• probably some other stuff that I forgot about.