docs: def set_bck_chunk(self, bck):
          """
          Sets the chunk backward from this chunk in the list of free chunks.
  
          :param bck: the chunk to precede this chunk in the list of free chunks
          """
          raise NotImplementedError("%s not implemented for %s" % (self.set_bck_chunk.__func__.__name__,
                                                                   self.__class__.__name__))