'Given a long verbnet class identifier (eg \'confess-37.10\'),
map it to a short id (eg \'37.10\').  If ``longid`` is already a
short id, then return it as-is.'

def shortid(self, longid):

    if self._SHORTID_RE.match(longid):
        return longid
    m = self._LONGID_RE.match(longid)
    if m:
        return m.group(2)
    else:
        raise ValueError((u'vnclass    identifier    %r    not    found' % longid))
