code: def open(self, options):
          
          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
docs: Open and include the refrenced schema.
          @param options: An options dictionary.
          @type options: L{options.Options}
          @return: The referenced schema.
          @rtype: L{Schema}