∀ {α : Type u_1} {β : Type u_2} {M : Type u_5} [inst : AddCommMonoid M] (s : Finset β) (f : α → β → M),
  (∀ b ∈ s, Function.HasFiniteSupport fun a => f a b) → ∑ᶠ (a : α), ∑ b ∈ s, f a b = ∑ b ∈ s, ∑ᶠ (a : α), f a b