*rkeys() {
        for (const i of this.#rindexes()) {
            const k = this.#keyList[i];
            if (k !== undefined &&
                !this.#isBackgroundFetch(this.#valList[i])) {
                yield k;
            }
        }
    }
Inverse order version of {@link LRUCache.keys}

Return a generator yielding the keys in the cache,
in order from least recently used to most recently used.