'Return the lemma for a word using WordNet\'s morphy function.
:param pos: Part of speech to filter upon. If `None`, defaults to
``_wordnet.NOUN``.
.. versionadded:: 0.8.1'

@requires_nltk_corpus
def lemmatize(self, pos=None):

    if (pos is None):
        pos = _wordnet.NOUN
    lemmatizer = nltk.stem.WordNetLemmatizer()
    return lemmatizer.lemmatize(self.string, pos)
