feat(data/set/basic): add lemmata (#2353)
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 <06bcd766296da4e7f3f1690c64fbdb3ee30e2f66@univ-rennes1.fr>

* change proof

Co-authored-by: sgouezel <06bcd766296da4e7f3f1690c64fbdb3ee30e2f66@univ-rennes1.fr>
Co-authored-by: mergify[bot] <b09a6ee808b67e98a221404e7aaa52e0398a4954@users.noreply.github.com>
leanprover-community/mathlib,leanprover-community/mathlib