proposition: Theorem second_fork {x y z w : C} (f : x ~> y) (g : x ~> z) (h : z ~> w) : second h ∘ f △ g ≈ f △ (h ∘ g).
proof: Proof. unfork. Qed.
imports: Category.Lib Category.Theory.Category Category.Theory.Isomorphism Category.Structure.Terminal