docs: def _closure_createlink(self):
          """Create a link in the closure tree."""
          linkparents = self._closure_model.objects.filter(
              child__pk=self._closure_parent_pk
          ).values("parent", "depth")
          linkchildren = self._closure_model.objects.filter(
              parent__pk=self.pk
          ).values("child", "depth")
          newlinks = [self._closure_model(
              parent_id=p['parent'],
              child_id=c['child'],
              depth=p['depth']+c['depth']+1
          ) for p in linkparents for c in linkchildren]
          self._closure_model.objects.bulk_create(newlinks)