'Return a list of all verb lemmas that appear in any class, or
in the ``classid`` if specified.'

def lemmas(self, classid=None):

    if (classid is None):
        return sorted(self._lemma_to_class.keys())
    else:
        vnclass = self.vnclass(classid)
        return [member.get(u'name') for member in vnclass.findall(u'MEMBERS/MEMBER')]
