feat(data/set/basic): add lemmata (#2353)
* feat(data/set/basic): add lemmata
* switch to term mode proof
* removing dupe
* make linter happy
* Update src/data/set/basic.lean
Co-Authored-By: sgouezel <sebastien.gouezel@univ-rennes1.fr>
* change proof
Co-authored-by: sgouezel <sebastien.gouezel@univ-rennes1.fr>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
