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kotlinlang

any idea why this syntax works
```
    override fun onAttachToView() {
        super.onAttachToView()
        subscriptions.add(view.sleepLaterClicks
                                  ?.debounce(250, TimeUnit.MILLISECONDS)
                                  ?.subscribe { view.navigateToSleepLater() })
        subscriptions.add(view.sleepNowClicks
                                  ?.debounce(250, TimeUnit.MILLISECONDS)
                                  ?.subscribe { view.navigateToSleepNow() })
    }
```

but this doesn't 
```
override fun onAttachToView() {
        super.onAttachToView()
        subscriptions.add(view.sleepLaterClicks?
                            .debounce(250, TimeUnit.MILLISECONDS)?
                            .subscribe { view.navigateToSleepLater() })
        subscriptions.add(view.sleepNowClicks?
                            .debounce(250, TimeUnit.MILLISECONDS)?
                            .subscribe { view.navigateToSleepNow() })
    }
```