∀ {R : Type u_1} [inst : AddCommGroup R] (f : ℤ → R) (n : ℕ),
  ∑ x ∈ Finset.Icc (-↑(n + 1)) ↑(n + 1), f x + ∑ x ∈ {-(↑(n + 1) + 1), ↑(n + 1) + 1}, f x =
    f (↑(n + 1) + 1) + f (-(↑(n + 1) + 1)) + ∑ m ∈ Finset.Icc (-↑(n + 1)) ↑(n + 1), f m