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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _registered(self):
 """ A optional boolean property indidcating whether this job store is registered. The registry is the authority on deciding if a job store exists or not. If True, this job store exists, if None the job store is transitioning from True to False or vice versa, if False the job store doesn't exist. :type: bool|None """

        # The weird mapping of the SDB item attribute value to the property value is due to
        # backwards compatibility. 'True' becomes True, that's easy. Toil < 3.3.0 writes this at
        # the end of job store creation. Absence of either the registry, the item or the
        # attribute becomes False, representing a truly absent, non-existing job store. An
        # attribute value of 'False', which is what Toil < 3.3.0 writes at the *beginning* of job
        # store destruction, indicates a job store in transition, reflecting the fact that 3.3.0
        # may leak buckets or domains even though the registry reports 'False' for them. We
        # can't handle job stores that were partially created by 3.3.0, though.
        registry_domain = self._bindDomain(domain_name='toil-registry',
                                           create=False,
                                           block=False)
        if registry_domain is None:
            return False
        else:
            for attempt in retry_sdb():
                with attempt:
                    attributes = registry_domain.get_attributes(item_name=self.namePrefix,
                                                                attribute_name='exists',
                                                                consistent_read=True)
                    try:
                        exists = attributes['exists']
                    except KeyError:
                        return False
                    else:
                        if exists == 'True':
                            return True
                        elif exists == 'False':
                            return None
                        else:
                            assert False