mysterious insufficient class constraints in Hugs-Jan98

[Byron Cook]

Hi, I get the following error when using Hugs (Jan'98) 
> ERROR "V.hs" (line 6): Insufficient class constraints in instance member
> binding
> *** Context  : ()
> *** Required : Anonymous a

on the following source file:
> data V2 a = V (a,a)
> 
> instance Vector V2 where
>   vector = V anon
>
> class Vector v where
>   vector :: Anonymous a => v a
>
> class Anonymous a where
>    anon :: a
>
> instance (Anonymous a,Anonymous b) => Anonymous (a,b) where
>    anon = (anon,anon)

GHC, HBC and Hugs-1.3c all have no problem with this file.  
HINT: it works in Yale-Hugs if you replace "vector = V anon" with 
"vector = V (anon,anon)".

- byron