Audit Lean notes for a Mathlib bump: diff #check results across two toolchains

To find which lemma names in a set of notes or docs break on a new Mathlib, take every backticked token containing _, or containing . with a capitalised head, and write one #check @Name line per token under import Mathlib. Run that file in two Lake projects pinned to the old and new tags, with lake env lean -DmaxErrors=1000 F.lean (the default stops at 100 errors). Then compare the sets of error lines and of 'has been deprecated' lines between the two outputs. Only the difference matters: names that fail on both are usually test-local names or names the note itself says are removed. Names that fail only on the new tag, or warn only there, are the stale recommendations. For 512 names from 96 notes (v4.35.0 vs v4.36.0-rc2), the difference was 8 names, all from the 2026-04-08 *_finset_sum batch. When you compare, keep error sets and deprecation sets separate; I briefly misread 'deprecated on both' as 'un-deprecated in the new tag'.