code: def open(self, options):
          
          if self.opened:
              return
          self.opened = True
          log.debug('%s, importing ns="%s", location="%s"',
                    self.id,
                    self.ns[1],
                    self.location
                    )
          result = self.locate()
          if result is None:
              if self.location is None:
                  log.debug('imported schema (%s) not-found', self.ns[1])
              else:
                  result = self.download(options)
          log.debug('imported:\n%s', result)
          return result
docs: Open and import the refrenced schema.
          @param options: An options dictionary.
          @type options: L{options.Options}
          @return: The referenced schema.
          @rtype: L{Schema}