'Returns the infinitive form of the given verb, or None.'

def lemma(self, verb, parse=True):

    if (dict.__len__(self) == 0):
        self.load()
    if (verb.lower() in self._inverse):
        return self._inverse[verb.lower()]
    if (verb in self._inverse):
        return self._inverse[verb]
    if (parse is True):
        return self.find_lemma(verb)
