docs: def chunk_from_mem(self, ptr):
          """
          Given a pointer to a user payload, return the chunk associated with that payload.
  
          :param ptr: a pointer to the base of a user payload in the heap
          :returns: the associated heap chunk
          """
          raise NotImplementedError("%s not implemented for %s" % (self.chunk_from_mem.__func__.__name__,
                                                                   self.__class__.__name__))