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