fundeps help

Hi, I'm trying to understand what fundeps do and don't let me do. One particular source of confusion is why the following program doesn't typecheck: {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-} module Fundeps where class Dep a b | a -> b, b -> a conv :: (Dep a b1,Dep a b2) => b1 -> b2 conv = id {- end of program -} It seems as if inferring that b1 = b2 is precisely what "improvement" is about, but I'm not really sure when GHC actually applies that. Is there any documentation of that? I've read Mark Jones' paper, the haskell-prime wiki entry about Fundeps and the GHC manual but am still rather lost. Cheers, Ganesh