A NubList is a list where all elements are unique, but are maintained in
their original order. The Monoid NubList's mappend implement is just:

    NubList xs `mappend` NubList ys = NubList . nub $ xs ++ ys

which preserves NubList's required properties (ordering and unique-ness
of elements) and meets the requirements of the Monoid laws (identity,
associativity and closure).