use default modifyLayout in Circle.

Circle.hs

@@ -35,7 +35,6 @@ data Circle a = Circle deriving ( Read, Show )
 instance Layout Circle Window where
     doLayout Circle r s = do layout <- raiseFocus $ circleLayout r $ integrate s
                              return (layout, Nothing)
-    modifyLayout Circle _ = return Nothing
 
 circleLayout :: Rectangle -> [a] -> [(a, Rectangle)]
 circleLayout _ []     = []