correction to example with Alfred ...

[S.D.Mechveliani]

I wrote on Fergus's example with Alfred, Betty ... 

                   type T a = T ...
                   instance Additive (T a) where
                                           (T x)+(T y) = T (x+y)
                   ...

It should be       data T a ...
                   instance Additive a => Additive (T a) where ...