simp [algebra.sum]
init₁ init₂ : ℕ
l₁ l₂ : List ℕ
h : List.length l₁ = List.length l₂
⊢ foldl algebra.sum (init₁ + init₂) (zipWith algebra.sum l₁ l₂) =
    algebra.sum (foldl algebra.sum init₁ l₁) (foldl algebra.sum init₂ l₂)
init₁ init₂ : ℕ
l₁ l₂ : List ℕ
h : List.length l₁ = List.length l₂
⊢ foldl (fun a b => a + b) (init₁ + init₂) (zipWith (fun a b => a + b) l₁ l₂) =
    foldl (fun a b => a + b) init₁ l₁ + foldl (fun a b => a + b) init₂ l₂