code: def _registered(self):
          
          # 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
docs: 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