'Return a list of all wordnet identifiers that appear in any
class, or in ``classid`` if specified.'

def wordnetids(self, classid=None):

    if (classid is None):
        return sorted(self._wordnet_to_class.keys())
    else:
        vnclass = self.vnclass(classid)
        return sum([member.get(u'wn', u'').split() for member in vnclass.findall(u'MEMBERS/MEMBER')], [])
