lowerUntil :: (Pretty a, Ord a, Ord b, Show a) =>
              (a -> b -> Bool) -- ^ cut-off predicate
           -> DepGraph a b c -- ^ dependency graph to be traversed
           -> [a] -- ^ compare entries to this element
           -> Set.Set (a, b)
lowerUntil _ _ [] = Set.empty
lowerUntil :: (Pretty a, Ord a, Ord b, Show a) =>
              (a -> b -> Bool) -- ^ cut-off predicate
           -> DepGraph a b c -- ^ dependency graph to be traversed
           -> [a] -- ^ compare entries to this element
           -> Set.Set (a, b)
lowerUntil _ _ [] = Set.empty
lowerUntil _ _ [] = Set.empty