proposition: Corollary nat_id_left C D (F G : C ⟶ D) (N : F ⟹ G) : nat_id ∙ N ≈[Fun] N.
proof: Proof. unfold nat_id, nat_compose; simpl; intros; cat. Qed.
imports: Category.Lib Category.Theory.Category Category.Theory.Isomorphism Category.Theory.Functor Category.Theory.Natural.Transformation Category.Instance.Sets