function subscribe(run, invalidate = noop) {
		/** @type {SubscribeInvalidateTuple<T>} */
		const subscriber = [run, invalidate];
		subscribers.add(subscriber);
		if (subscribers.size === 1) {
			stop = start(set, update) || noop;
		}
		run(/** @type {T} */ (value));
		return () => {
			subscribers.delete(subscriber);
			if (subscribers.size === 0 && stop) {
				stop();
				stop = null;
			}
		};
	}
@param {Subscriber<T>} run
@param {() => void} [invalidate]
@returns {Unsubscriber}