∀ {α : Type u_1} {β : Type u_2} {L : SummationFilter β} [inst : AddCommGroup α] [inst_1 : TopologicalSpace α]
  [IsTopologicalAddGroup α] {f : β → α} [T2Space α], -∑'[L] (b : β), f b = ∑'[L] (b : β), -f b