How to -retry: only matched error?

How can I ```-retry:``` only some type of error?

With this patch, you can retry on time-out error only.
```
[signal retry:3 matchingError:^BOOL(NSError *error) {
    return [error.domain compare:NSURLErrorDomain] == 0 && error.code == NSURLErrorTimedOut;
}];
```

How do you think? Is there another recommended way without patch the core operators?