set(key1, key2, value) {
		let childMap = this._map.get(key1);
		if (childMap === undefined) {
			childMap = new WeakMap();
			this._map.set(key1, childMap);
		}
		childMap.set(key2, value);
	}
@param {K1} key1 first key
@param {K2} key2 second key
@param {T | undefined} value new value
@returns {void}