proposition: Corollary inl_right {x y z : C} (f : x ~> y) : right f ∘ @inl z x ≈ inl.
proof: Proof. apply (@exl_second _ O). Qed.
imports: Category.Lib Category.Theory.Category Category.Theory.Isomorphism Category.Structure.Cartesian Category.Structure.Initial Category.Construction.Opposite