:: Ord a => [a] -> [a] -package:hspec -package:mono-traversable -package:containers -package:imports is:exact -package:statistics -package:basement 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]