lowerLevel :: Ord a => DepGraph a b c -> [a] -> Set.Set a
lowerLevel gr = Set.unions . mapMaybe f where
    f a = fmap g $ Map.lookup a $ dataMap gr
    g (_, dss, _) = dss
lowerLevel :: Ord a => DepGraph a b c -> [a] -> Set.Set a
lowerLevel gr = Set.unions . mapMaybe f where
    f a = fmap g $ Map.lookup a $ dataMap gr
    g (_, dss, _) = dss
lowerLevel gr = Set.unions . mapMaybe f where
    f a = fmap g $ Map.lookup a $ dataMap gr
    g (_, dss, _) = dss