'Open and include the refrenced schema.
@param options: An options dictionary.
@type options: L{options.Options}
@return: The referenced schema.
@rtype: L{Schema}'

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
