Overwrite implementation of show function - comp.lang.haskell

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From: Frank Poettgen <Frank.Poett...@Post.RxWyTxH-AyAzCyHzEyN.de>
Newsgroups: comp.lang.haskell
Subject: Overwrite implementation of show function
Date: Thu, 28 May 2009 19:35:47 +0200
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import Data.Set as Set ( Set, fromList, toList )

test = fromList [fromList [1,2],fromList [3]]

Then 'show test' will result in the following string:
"fromList [fromList [1,2],fromList [3]]"

What I want is something like "[[1,2],[3]]"
Is there a way to overwrite the implementation of the show function?

instance (Show a) => Show (Set a) where
   show x = toList x

-> ERROR Overlapping instances for class "Show"