'Open and import 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,    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
