mathlib

ad9848f1 - feat(data/finsupp/basic): add `finsupp.not_mem_erase`

feat(data/finsupp/basic): add `finsupp.not_mem_erase` Adding `finsupp.not_mem_erase` to avoid an issue discussed in this Zulip thread: https://leanprover.zulipchat.com/#narrow/stream/113489-new-members/topic/finsupp.2Esupport_erase