:: Ord a => [a] -> [a] -package:statistics -package:safe -package:basement -package:Cabal -package:imports -package:containers package:witherable -is:exact

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]