'Determine an appropriate tag for the specified token, and
return that tag.  If this tagger is unable to determine a tag
for the specified token, then its backoff tagger is consulted.
:rtype: str
:type tokens: list
:param tokens: The list of words that are being tagged.
:type index: int
:param index: The index of the word whose tag should be
returned.
:type history: list(str)
:param history: A list of the tags for all words before *index*.'

def tag_one(self, tokens, index, history):

    tag = None
    for tagger in self._taggers:
        tag = tagger.choose_tag(tokens, index, history)
        if (tag is not None):
            break
    return tag
