fact: Definition doppel {x y z : C} : (x × y) × z ~> (x × z) × (y × z) := first exl △ first exr.
imports: Category.Lib Category.Theory.Category Category.Theory.Isomorphism Category.Theory.Functor Category.Functor.Bifunctor Category.Structure.Terminal Category.Structure.Cartesian Category.Structure.Monoidal Category.Structure.Monoidal.Internal.Product Category.Structure.Cartesian.Closed Category.Instance.Sets