class instance with nested types

Hello, I have the following problem: basic datatypes

type Sequence a = [a] data Tree a = N a (Forest a) deriving (Ord,Eq,Show) type Forest a = Sequence (Tree a)

i want to construct a class Xy

class Xy s a where test :: s a -> a

and make an instance for list of characters

instance Xy [] Char where test [a] = a

this works, and an instance for a forest and tried something like this

instance ([] Tree) Char where test x@(N a xs):txs = a

I get illegal type errors. Is it possible to use nested types in a class ? Hope you can help me Matthias