Functor D (C + D) := Build_Functor D (C + D) (@ _ _) (fun _ _ m => m) (fun _ _ _ _ _ => idpath) (fun _ => idpath).
Require Import Category.Sum Functor.Core Functor.Composition.Core Functor.Identity. Require Import Functor.Paths HoTT.Tactics Types.Forall. Require Import Basics.Tactics.