[xmonad] Testing GHC HEAD

[Carsten Mattner]

xmonad built fine with GHC HEAD but XMonadContrib failed with the following
build error:

XMonad/Layout/MultiToggle.hs:162:9:
    Could not deduce (HList b w0)
      arising from the ambiguity check for `??'
    from the context (HList b w)
      bound by the type signature for
                 ?? :: HList b w => a -> b -> HCons a b
      at XMonad/Layout/MultiToggle.hs:162:9-42
    The type variable `w0' is ambiguous
    In the ambiguity check for:
      forall a b w. HList b w => a -> b -> HCons a b
    In the type signature for `??':
      ?? :: HList b w => a -> b -> HCons a b

Can anyone suggest a fix?