def get_constituency_extractor(self):
        """
        Returns a constituency extractor object
        @rtype: L{Cconstituency_extractor}
        @return: a constituency extractor object
        """

        if self.constituency_layer is not None:    ##Otherwise there are no constituens
            if self.my_constituency_extractor is None:
                self.my_constituency_extractor = Cconstituency_extractor(self)
            return self.my_constituency_extractor
        else:
            return None
Returns a constituency extractor object
        @rtype: L{Cconstituency_extractor}
        @return: a constituency extractor object