rintro ⟨h, h0, h1⟩
case mpr
f : ℕ → ℕ
⊢ ((∀ (x y : ℕ), f (y + f x) ≤ f y + x) ∧ (∀ (x y : ℕ), x ≤ f y + f (y + f x)) ∧ ∀ (x y : ℕ), f y ≤ f (y + f x) + x) →
    f = fun x => x
case mpr.intro.intro
f : ℕ → ℕ
h : ∀ (x y : ℕ), f (y + f x) ≤ f y + x
h0 : ∀ (x y : ℕ), x ≤ f y + f (y + f x)
h1 : ∀ (x y : ℕ), f y ≤ f (y + f x) + x
⊢ f = fun x => x