'Return the parse tree for the sentence.'

def parse(self, sentence):

    if (not self._trained):
        self.train()
    pos_tags = [pos for (word, pos) in sentence]
    tagged_pos_tags = self.tagger.tag(pos_tags)
    chunktags = [chunktag for (pos, chunktag) in tagged_pos_tags]
    conlltags = [(word, pos, chunktag) for ((word, pos), chunktag) in zip(sentence, chunktags)]
    return nltk.chunk.util.conlltags2tree(conlltags)
