src/vs/workbench/contrib/audioCues/browser/observable.ts/LazyDerivedObserver/handleBeforeReadObservable
class LazyDerivedObserver:	public handleBeforeReadObservable<T>(observable: IObservable<T>) {
		this._dependencies.add(observable);
		if (!this.staleDependencies.delete(observable)) {
			observable.subscribe(this);
		}
	}