## Recall: System F
```agda
ℕ : Set₁
ℕ = ∀ {A} → (A → A) → A → A
zero : ℕ
zero = λ f x → x
suc : ℕ → ℕ
suc = λ n f x → f (n f x)
fold : ∀ {A} → (A → A) → A → ℕ → A
fold = λ f x n → n f x
-- add : ℕ → ℕ → ℕ
-- add = λ n → fold suc n
```