fact: Fixpoint concat `(xs : t (t a n) m) : t a (m * n) := match xs with | nil _ => nil a | cons _ hd n tl => append hd (concat tl) end.
imports: Category.Lib Category.Theory.Category Category.Theory.Isomorphism Category.Theory.Functor Category.Theory.Adjunction Category.Structure.Terminal Category.Structure.Cartesian Category.Structure.BiCCC Category.Instance.Fun Category.Instance.Coq Category.Instance.Sets Coq.Vectors.Vector