∀ {ι : Type u_1} {M : Type u_3} {γ : Type u_5} [inst : AddCommMonoid M] {s : Finset ι} {p : ι → Prop}
  [inst_1 : DecidablePred p] (f g : ι → γ) (h : γ → M),
  ∑ x ∈ s, h (if p x then f x else g x) = ∑ x ∈ s with p x, h (f x) + ∑ x ∈ s with ¬p x, h (g x)