Disjoint type classes

[Jose Emilio Labra Gayo]

I have a large program where I would like to include disjoint type classes
into a new type class.

My problem can be reduced to the following example (taken from [1]):

> class Num a => Dividable a
>   where dividedBy :: a -> a -> a

> instance Fractional a => Dividable a where
> dividedBy = (/)

> instance Integral a => Dividable a where
>   dividedBy = div

Which doesn't work because Haskell doesn't detect that
"Integral" and "Fractional" are disjoint.

Is there a way to implement these type classes with current Haskell
implementations?

[1] K. Glynn, M. Sulzmann,  P.J. Stuckey
    Type Classes and Constraint Handling Rules
    http://www.cs.mu.oz.au/tr_submit/test/cover_db/mu_TR_2000_7.html

Best regards, Jose Labra