∀ {ι : Type u_1} {α : ι → Type u_3} {δ : (i : ι) → α i → Type u_2} [inst : DecidableEq ι]
  [inst_1 : (i : ι) → (j : α i) → AddCommMonoid (δ i j)] (x y : DirectSum ι fun i => DirectSum (α i) fun j => δ i j),
  (↑DirectSum.sigmaUncurry).toFun (x + y) = (↑DirectSum.sigmaUncurry).toFun x + (↑DirectSum.sigmaUncurry).toFun y