@requires_nltk_corpus<EOL><INDENT>def lemmatize(self, pos=None):<DEDENT>
<EOL>raise NotImplementedError<EOL>
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 (``textblob``)