[Haskell] How to close a type class

[Keean Schupke]

Just spotted this typo:

>
> How can we prevent the user from adding instances to Fail, whilst 
> still exporting
> Fail so that it can be used in the constraints of other classes, like:
>
>     class MustBeInt a
>     instance MustBeInt a

    instance MustBeInt Int

>     instance Fail a => MustBeInt a

    Keean