mathlib3

cd424234 - feat(topology/algebra/infinite_sum): add tsum_finset_bUnion_disjoint (#18350)

feat(topology/algebra/infinite_sum): add tsum_finset_bUnion_disjoint (#18350) This is the n-ary version of `tsum_union_disjoint`, although unfortunately I realized that what I actually wanted was the n-ary version of `has_sum.add_is_compl` which is harder to state! Either way, this lemma seems worth having.