mathlib

f13e5dfd - refactor(set_theory/*) rename `wf` lemmas to `lt_wf` (#14417)

refactor(set_theory/*) rename `wf` lemmas to `lt_wf` (#14417) This is done for consistency with the rest of `mathlib` (`nat.lt_wf`, `enat.lt_wf`, `finset.lt_wf`, ...)