nub.hs

bun []                   = []
bun (x:xs) | x `elem` xs =     bun xs
           | otherwise   = x : bun xs

nub :: Eq a => [a] -> [a]
nub = reverse . bun . reverse

nub_fragile.pl

append3(XS, YS, ZS, OS) :- append(XS, WS, OS), append(YS, ZS, WS).

% nub(PS ++ (Q:QS) ++ (Q:RS)) = nub(PS ++ (Q:QS) ++ RS)
nub(XS, OS) :- append3(PS, [Q|QS], [Q|RS], XS), !,
               append3(PS, [Q|QS], RS, YS),
               nub(YS, OS).
nub(XS, XS).