Previous passage Identifier

        :rtype: CtsPassage
        :returns: Previous passage at same level

def prevId(self):
        """ Previous passage Identifier

        :rtype: CtsPassage
        :returns: Previous passage at same level
        """
        if self._prev_id is False:
            # Request the next urn
            self._prev_id, self._next_id = self.getPrevNextUrn(reference=self.urn.reference)
        return self._prev_id