theorem extracted_formal_statement_28 {n : ℕ} (f : ℤ → Signs n → ℝ) :
  ∑ σ' ∈ {-1, 1}, ∑ σ, f σ' σ = ∑ σ, f (↑σ.1) σ.2 := sorry

State before:
n : ℕ
f : ℤ → Signs n → ℝ
⊢ ∑ σ' ∈ {-1, 1}, ∑ σ, f σ' σ = ∑ σ, f (↑σ.1) σ.2

Tactic: exact Eq.symm (Fintype.sum_prod_type _)

State after:

Relevant lemmas:
Fintype.sum_prod_type : ∀ {γ : Type u_3} {α₁ : Type u_4} {α₂ : Type u_5} [inst : Fintype α₁] [inst_1 : Fintype α₂]
  [inst_2 : AddCommMonoid γ] (f : α₁ × α₂ → γ), ∑ x, f x = ∑ x, ∑ y, f (x, y)
Eq.symm : ∀ {α : Sort u} {a b : α}, a = b → b = a