docs: def everything(self, content_id=None):
          '''Returns a generator of all labels in the store.
  
          If ``content_id`` is set, will restrict results to labels
          containing the provided ``content_id``.
          '''
          if content_id is not None:
              ranges = [((content_id,), (content_id,))]
          else:
              ranges = []
  
          labels = self.kvl.scan(self.TABLE, *ranges)
          labels = imap(lambda p: self._label_from_kvlayer(*p), labels)
          return labels