*values() {
        for (const i of this.#indexes()) {
            const v = this.#valList[i];
            if (v !== undefined &&
                !this.#isBackgroundFetch(this.#valList[i])) {
                yield this.#valList[i];
            }
        }
    }
Return a generator yielding the values in the cache,
in order from most recently used to least recently used.