:: Ord a => [a] -> [a] -package:html-entities -package:lens -package:imports -package:base is:exact -package:containers -package:hedgehog -package:incipit-base 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]