mathlib3

899bb5f9 - feat(data/multiset): `(s.erase x).map f = (s.map f).erase (f x)` (#8375)

feat(data/multiset): `(s.erase x).map f = (s.map f).erase (f x)` (#8375) A little lemma that I needed for Dedekind domains. Co-authored-by: Anne Baanen <Vierkantor@users.noreply.github.com>