-->
Subtypes: Opted, Maybe, Stops, Boolean.
Sum is a colimit of Arrow category:
Covariant Endo Functor from Arrow into Arrow (1/2 argument):
Covariant Endo Functor from Arrow into Arrow (2/2 argument):
Covariant Endo Functor from Kleisli Arrow into Kleisli Arrow (1/2 argument):
Covariant Endo Functor from Kleisli Arrow into Kleisli Arrow (2/2 argument):