[xmonad] mouse buttons "button8" and "button9" don't wok

Hi, I'm using mouse bindings and want to add my "forward" and "back" buttons, but get an "not in scope" error when I do so. Here is my config peace: myMouseBindings (XConfig {XMonad.modMask = modMask}) = M.fromList $ [ ((modMask, button1), (\w -> focus w >> mouseMoveWindow w >> windows W.shiftMaster)) -- set the window to floating mode and move by dragging , ((modMask, button2), (\w -> focus w >> windows W.shiftMaster)) -- raise the window to the top of the stack -- , ((modMask, button9), (toggleWS)) -- switch to next workspace ] kind regards, Stefan