rxjsで購読を止める

色々やり方はあると思うが一つ聞いたのはこういうもの。

const root = new BehaviorSubject<"start"|"end">("start");
const subject = new Subject<string>();
const op = root.pipe(mergeMap((x) => {
    if (x == "end") {
        return throwError(new Error("end"));
    }
    return subject;
})).pipe()
op.subscribe((x) => {
    console.log("1:"+x);
},(e) => {
    console.log("1:"+e);
})
subject.next("a");
subject.next("b");
root.next("end");
root.next("start");
op.subscribe((x) => {
    console.log("2:"+x);
},(e) => {
    console.log("2:"+e);
})
subject.next("c");
subject.next("d");
root.next("end");
root.next("start");
subject.next("e");
subject.next("f");
/*
1:a
1:b
1:Error: end
2:c
2:d
2:Error: end
*/