∀ {α : Type u_1} {β : Type u_2} [inst : AddCommMonoid α] [inst_1 : TopologicalSpace α] {f : β → α} [T2Space α]
  [ContinuousAdd α] {s t : Set β},
  Disjoint s t →
    Summable (f ∘ Subtype.val) →
      Summable (f ∘ Subtype.val) → ∑' (x : ↑(s ∪ t)), f ↑x = ∑' (x : ↑s), f ↑x + ∑' (x : ↑t), f ↑x