mathlib

f0451d80 - feat(algebra/group/defs): ext lemmas for (semi)groups and monoids (#8391)

feat(algebra/group/defs): ext lemmas for (semi)groups and monoids (#8391) [Zulip discussion](https://leanprover.zulipchat.com/#narrow/stream/144837-PR-reviews/topic/.236476.20boolean_algebra.2Eto_boolean_ring/near/242118386) Co-authored-by: Kevin Buzzard <k.buzzard@imperial.ac.uk>