docs: def unfinished_objects(self):
          '''
          Leaves only versions of those objects that has some version with
          `_end == None` or with `_end > right cutoff`.
          '''
          mask = self._end_isnull
          if self._rbound is not None:
              mask = mask | (self._end > self._rbound)
          oids = set(self[mask]._oid.tolist())
          return self[self._oid.apply(lambda oid: oid in oids)]