Instead of returning `nil`, I think we should be returning a `RACDisposable` that cancels the operation:

```objective-c
- (RACSignal *)createAccountForEmail:(NSString *)email
                           firstName:(NSString *)firstName
                            lastName:(NSString *)lastName
{
    return [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
        AFHTTPRequestOperation *operation = [requestManager
            POST:@"/accounts"
            parameters:@{ @"first_name": firstName, @"last_name": lastName, @"email": email, }
            success:^(AFHTTPRequestOperation *operation, id responseObject) {
                [subscriber sendNext:responseObject];
                [subscriber sendCompleted];
            }
            failure:^(AFHTTPRequestOperation *operation, NSError *error) {
                [subscriber sendError:error];
            }];
        return [RACDisposable disposableWithBlock:^{
            [operation cancel];
        }];
    }];
}
```

(There's a good discussion on this in AFNetworking-RACExtensions#8).