proposition: Theorem cover_inl {x y z w : C} (f : x ~> y) (g : z ~> w): cover f g ∘ inl ≈ inl ∘ f.
proof: Proof. unmerge; cat. Qed.
imports: Category.Lib Category.Theory.Category Category.Theory.Isomorphism Category.Structure.Cartesian Category.Structure.Initial Category.Construction.Opposite