(** Opposite of a functor F : C -> D is a functor F^op : C^op -> D^op with the same object and arrow maps. *)
Section Opposite_Functor.
Context {C D : Category} (F : (C –≻ D)%functor).
Local Open Scope morphism_scope.
Local Open Scope object_scope.
Program Definition Opposite_Functor : (C^op –≻ D^op)%functor :=
{|
FO := F _o;
FA := fun _ _ h => F @_a _ _ h;
F_id := fun a => F_id F a;
F_compose := fun _ _ _ f g => F_compose F g f
|}.
End Opposite_Functor.
Notation "F '^op'" := (Opposite_Functor F) : functor_scope.
(* We can compose functors. The object and arrow maps are simply function compositions of object and arrow maps. *)
Section Functor_Compose.
Context {C C' C'' : Category} (F : (C –≻ C')%functor) (F' : (C' –≻ C'')%functor).
Local Open Scope morphism_scope.
Local Open Scope object_scope.
Program Definition Functor_compose : (C –≻ C'')%functor :=
{|
FO := fun c => F' _o (F _o c);
FA := fun c d f => F' _a (F _a f)
|}.
End Functor_Compose.
Notation "F ∘ G" := (Functor_compose G F) : functor_scope.
(** Associativity of functor composition *)
Section Functor_Assoc.
Context {C1 C2 C3 C4 : Category}
(F : (C1 –≻ C2)%functor)
(G : (C2 –≻ C3)%functor)
(H : (C3 –≻ C4)%functor).
Local Open Scope functor_scope.

Theorem Functor_assoc : (H ∘ G) ∘ F = H ∘ (G ∘ F).
End Functor_Assoc.