mergeIM :: (Int->a->Maybe b->c) -> IM.IntMap a -> IM.IntMap b -> IM.IntMap c
mergeIM = case True of
    True  -> mergeIM_s
    False -> mergeIM_f

mergeIM_s :: (Int->a->Maybe b->c) -> IM.IntMap a -> IM.IntMap b -> IM.IntMap c
mergeIM_s f im im' = IM.mapWithKey g im
  where
    g k x = f k x $ IM.lookup k im'

mergeIM_f :: (Int->a->Maybe b->c) -> IM.IntMap a -> IM.IntMap b -> IM.IntMap c
mergeIM_f f = IM.mergeWithKey mrg (IM.mapWithKey lft) (const IM.empty)
  where
    mrg i x y = Just $ f i x $ Just y

    lft i x   = f i x Nothing