src/vs/base/common/observableImpl/base.ts/ConvenientObservable/map
class ConvenientObservable:	public map<TNew>(fn: (value: T) => TNew): IObservable<TNew> {
		return _derived(
			() => {
				const name = getFunctionName(fn);
				return name !== undefined ? name : `${this.debugName} (mapped)`;
			},
			(reader) => fn(this.read(reader))
		);
	}