replace h := h 1
case mpr.intro.intro
f : ℕ → ℕ
h : ∀ (x y : ℕ), f (y + f x) ≤ f y + x
h1 : f 0 = 0
h0 : ∀ (x : ℕ), f (f x) = x
⊢ f = fun x => x
case mpr.intro.intro
f : ℕ → ℕ
h1 : f 0 = 0
h0 : ∀ (x : ℕ), f (f x) = x
h : ∀ (y : ℕ), f (y + f 1) ≤ f y + 1
⊢ f = fun x => x