[don't use mapMonotonic in domain in case there are duplicate entries
John Meacham <john@repetae.net>**20060425051757] hunk ./Util/Relation.hs 12
-domain :: Ord a => Rel a b -> Set a
-domain r = mapMonotonic fst r
+domain :: (Ord a,Ord b) => Rel a b -> Set a
+domain r = Set.map fst r