Commit 2022-07-21 16:19 f05fdcac
View on Github →refactor(set_theory/zfc): make Class morally Set → Prop (#15248)
We use Class in place of Set → Prop (within the Class API), and document this decision.
Note that there's no longer much reason to have Class.to_Set separately from Class.mem. I will suggest inlining both into the has_mem instance in a followup PR.
See Zulip).