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