∀ {α : Type u_1} {β : Type u_2} [inst : AddCommMonoid α] [inst_1 : TopologicalSpace α] {L : SummationFilter β}
  (P : Prop) [inst_2 : Decidable P] (x : β → ¬P → α),
  (if h : P then 0 else ∑'[L] (b : β), x b h) = ∑'[L] (b : β), if h : P then 0 else x b h