overlapping instances

[Arie Peterson]

>What would have GHC do for the "convert :: String -> String" case?
>(You'd like "convert = id" here but it's not clear to me what the
>general rule should be.)
>
>         mike

The general rule should be that convert :: a -> a = id, no matter what 
other instances are available.
Is there any way to tell GHC so? It would be nice to be able to define 
'precedences'.

Arie