title: RxSwift how to append to BehaviorSubject<[]>
body: <p>Since Variable is deprecated in RxSwift 4, what is the equivalent way for <code>BehaviorSubject</code> to do the following?</p>
  
  <pre><code>let observable = Variable&lt;[Int]&gt;([])
  observable.value.append(1)
  </code></pre>