rcases h0 with rfl | rfl | rfl
case mpr
R : Type u_1
D : Type u_2
inst✝ : DivisionRing D
f✝ : D → D
h✝ : good f✝
h : ∀ (f : D → D), good f → f 0 = 1 → Injective f
f : D → D
h0 : f = 0 ∨ (f = fun x => 1 - x) ∨ f = -fun x => 1 - x
⊢ good f
case mpr.inl
R : Type u_1
D : Type u_2
inst✝ : DivisionRing D
f : D → D
h✝ : good f
h : ∀ (f : D → D), good f → f 0 = 1 → Injective f
⊢ good 0

case mpr.inr.inl
R : Type u_1
D : Type u_2
inst✝ : DivisionRing D
f : D → D
h✝ : good f
h : ∀ (f : D → D), good f → f 0 = 1 → Injective f
⊢ good fun x => 1 - x

case mpr.inr.inr
R : Type u_1
D : Type u_2
inst✝ : DivisionRing D
f : D → D
h✝ : good f
h : ∀ (f : D → D), good f → f 0 = 1 → Injective f
⊢ good (-fun x => 1 - x)