ntr :: Eq a => [[a]] -> [[a]]
ntr = foldr (\x acc -> ntr' x acc) []
ntr' x [] = [x]
ntr' x (y:ys) = case x `intersect` y of
  [] -> y : ntr' x ys
  _  -> (x `union` y) : ys

test = [[(1,2),(1,3),(2,1)],[(1,2),(1,3),(2,3)],[(1,2),(2,1)],[(1,3),(2,3),(3,2)],[(2,3),(3,2),(4,2)],[(2,5),(3,4)],[(2,5),(3,4),(4,3)],[(3,2),(4,2),(4,3)],[(3,4),(4,2),(4,3)]]

ntr test = [[(1,2),(1,3),(2,1),(2,3),(3,2),(4,2),(2,5),(3,4),(4,3)],[(1,2),(2,1)]]