(C D : PreCategory) : Functor (C -> D) (C^op -> D^op)^op := Build_Functor (C -> D) ((C^op -> D^op)^op) (fun F => F^op)%functor (fun _ _ T => T^op)%natural_transformation (fun _ _ _ _ _ => idpath) (fun _ => idpath).
Require Import Category.Core Functor.Core NaturalTransformation.Core. Require Import Category.Dual Functor.Dual NaturalTransformation.Dual. Require Import Functor.Identity. Require Import FunctorCategory.Core. Require Import Functor.Paths. Require Import HoTT.Tactics.