:: Ord a => [a] -> [a] -package:focus -package:safe -package:containers -package:base -package:errors -package:extra -package:Cabal -package:incipit-base is:exact 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]