In addition to UnigramTagger, there are two more NgramTagger subclasses:
BigramTagger and TrigramTagger. The BigramTagger subclass uses the previous tag as
part of its context, while the TrigramTagger subclass uses the previous two tags. An ngram
is a subsequence of n items, so the BigramTagger subclass looks at two items (the previous
tagged word and the current word), and the TrigramTagger subclass looks at three items.