suffices (f.invFun ∘ f) x = x by assumption
α : Sort u_1
β : Sort u_2
inst✝ : Nonempty α
f : α → β
hf : Injective f
x : α
⊢ invFun f (f x) = x
α : Sort u_1
β : Sort u_2
inst✝ : Nonempty α
f : α → β
hf : Injective f
x : α
⊢ (invFun f ∘ f) x = x