∀ {α : Type u_1} {ι : Type u_3} {𝔖 : Set (Set α)} {β : Type u_4} [inst : AddCommMonoid β] {f : ι → α → β}
  (I : Finset ι), (UniformOnFun.toFun 𝔖) (∑ i ∈ I, f i) = ∑ i ∈ I, (UniformOnFun.toFun 𝔖) (f i)