fact: Definition cover {x y z w : C} (f : x ~> y) (g : z ~> w) : x + z ~{C}~> y + w := (inl ∘ f) ▽ (inr ∘ g).
imports: Category.Lib Category.Theory.Category Category.Theory.Isomorphism Category.Structure.Cartesian Category.Structure.Initial Category.Construction.Opposite