addToMapOfItems = (map, key, value) => {
	let array = map.get(key);
	if (array === undefined) {
		array = [];
		map.set(key, array);
	}
	array.push(value);
}
@template K
@template V
@param {Map<K, V[]>} map a map from key to values
@param {K} key key
@param {V} value value
@returns {void}