how to prevent the second observable to execute?

Thanks.

```
      Observable<Data> source = Observable.concat(
                sources.memory(),
                sources.disk(),
                sources.network()
            )
            .first(data -> data != null && data.isUpToDate());
```

I want to know when the `first()`'s condition is true, is there any way to present the following `observable` to execute?

Thank you.
