Bug with fun-deps

Hello,

I seem to have run into a problem involving functional dependencies.
Below is an excerpt of a program I was writing that illustrates the bug.

> data N0
> newtype Succ n    = Succ n

> class Plus a b c | a b -> c
> instance Plus N0 n n
> instance Plus a b c => Plus (Succ a) b (Succ c)

> ( # )              :: Plus x y z => x -> y -> z
> ( # )               = undefined

> class BitRep t n | t -> n where
>   toBits         :: t -> n

> instance BitRep Bool (Succ N0)

> instance BitRep (Bool,Bool,Bool) (Succ (Succ (Succ N0))) where
>   toBits (x,y,z) =

The program works with the line below, but if I remove it
I get a type error --- Hugs complains that it cannot solve
the constraint:

Plus (Succ N0) (Succ N0) (Succ (Succ N0))

>     toBits' where toBits' =

>                         toBits x
>                       # toBits y
>                       # toBits z

The program works with GHC in either form, and it looks like
Hugs should be able to solve the constraint using the instances above.

-Iavor