proposition: Theorem left_right {x y z w : C} (f : x ~> y) (g : z ~> w) : left f ∘ right g ≈ right g ∘ left f.
proof: Proof. symmetry; apply (@first_second _ O). Qed.
imports: Category.Lib Category.Theory.Category Category.Theory.Isomorphism Category.Structure.Cartesian Category.Structure.Initial Category.Construction.Opposite