_addInternal(key, item) {
		let entry = this._map.get(key);
		if (entry === undefined) {
			entry = this._leaf
				? new SortableSet(
						undefined,
						/** @type {Comparator<T>} */
						(this._innerArgs[0])
					)
				: new LazyBucketSortedSet(
						.../** @type {[GetKey<T, K>, Comparator<K>]} */
						(this._innerArgs)
					);
			this._keys.add(key);
			this._map.set(key, entry);
		}
		entry.add(item);
	}
@param {K} key key of item
@param {T} item the item
@returns {void}