'Adds a sentence to the corpus.'

def _add_sentence(self, tags, heads, labels, corpus):

    sentence = sentence_pb2.Sentence()
    for (tag, head, label) in zip(tags, heads, labels):
        sentence.token.add(word='x', start=0, end=0, tag=tag, head=head, label=label)
    corpus.append(sentence.SerializeToString())
