static create(items, additionalItems) {
        if (items <= 128 && additionalItems.length === 0) {
            // We create a cache of 128=2^7 elements to cover all sets with up to 7 (dense) elements.
            let cached = SmallImmutableSet.cache[items];
            if (!cached) {
                cached = new SmallImmutableSet(items, additionalItems);
                SmallImmutableSet.cache[items] = cached;
            }
            return cached;
        }
        return new SmallImmutableSet(items, additionalItems);
    }
Represents an immutable set that works best for a small number of elements (less than 32).
It uses bits to encode element membership efficiently.