funext 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
h2 : f 1 = 1
⊢ f = fun x => x
case mpr.intro.intro.h
f : ℕ → ℕ
h1 : f 0 = 0
h0 : ∀ (x : ℕ), f (f x) = x
h : ∀ (y : ℕ), f (y + f 1) ≤ f y + 1
h2 : f 1 = 1
x : ℕ
⊢ f x = x