Sum is a colimit of Arrow category:
Covariant Functor from Arrow into Arrow (1/2 argument):
Covariant Functor from Arrow into Arrow (2/2 argument):
Covariant Functor from Kleisli Arrow into Kleisli Arrow (1/2 argument):
Covariant Functor from Kleisli Arrow into Kleisli Arrow (2/2 argument):