:: Ord a => [a] -> [a] -package:html-entities -package:incipit-base -package:statistics -package:containers -package:errors -package:safe -package:Cabal package:witherable

Removes duplicate elements from a list, keeping only the first occurrence. This is asymptotically faster than using nub from Data.List.
>>> ordNub [3,2,1,3,2,1]
[3,2,1]