def open(self, options):
        """
        Open and include the refrenced schema.
        @param options: An options dictionary.
        @type options: L{options.Options}
        @return: The referenced schema.
        @rtype: L{Schema}
        """
        if self.opened:
            return
        self.opened = True
        log.debug('%s, including location="%s"', self.id, self.location)
        result = self.download(options)
        log.debug('included:\n%s', result)
        return result