mathlib

d610cec7 - feat(group_theory/quotient_group): `simp` lemmas for `quotient_group.map` (#16703)

feat(group_theory/quotient_group): `simp` lemmas for `quotient_group.map` (#16703) Little lemmas that I needed to work with the class group of a ring of integers. Co-authored-by: Anne Baanen <Vierkantor@users.noreply.github.com>