docs: def _remove_add_key(self, key):
          """Move a key to the end of the linked list and discard old entries."""
          if not hasattr(self, '_queue'):
              return  # haven't initialized yet, so don't bother
          if key in self._queue:
              self._queue.remove(key)
          self._queue.append(key)
          if self.maxsize == 0:
              return
          while len(self._queue) > self.maxsize:
              del self[self._queue[0]]