O(n^2). The
nub function removes duplicate elements from
a list. In particular, it keeps only the first occurrence of each
element. (The name
nub means `essence'.) It is a special case
of
nubBy, which allows the programmer to supply their own
equality test.
>>> nub [1,2,3,4,3,2,1,2,4,3,5]
[1,2,3,4,5]