create default modifyLayout that ignores messages.

Operations.hs

@@ -381,7 +381,6 @@ instance Message IncMasterN
 data Full a = Full deriving ( Show, Read )
 instance Layout Full a where
     doLayout Full sc (W.Stack f _ _) = return ([(f, sc)], Nothing)
-    modifyLayout Full _ = return Nothing -- no changes
 --
 -- The tiling mode of xmonad, and its operations.
 --

XMonad.hs

@@ -152,6 +152,7 @@ readLayout ls s = concatMap rl ls
 class (Show (layout a), Read (layout a)) => Layout layout a where
     doLayout :: layout a -> Rectangle -> Stack a -> X ([(a, Rectangle)], Maybe (layout a))
     modifyLayout :: layout a -> SomeMessage -> X (Maybe (layout a))
+    modifyLayout _ _ = return Nothing
 
 runLayout :: Layout l a => l a -> Rectangle -> StackOrNot a -> X ([(a, Rectangle)], Maybe (l a))
 runLayout l r = maybe (return ([], Nothing)) (doLayout l r)