title: RxSwift merge different kind of Observables
body: <p>How should I merge 2 different types of <code>Observable</code>s in RxSwift?</p>
  
  <p>For example:</p>
  
  <pre><code>var a: Observable&lt;Int&gt;
  var b: Observable&lt;Void&gt;
  </code></pre>
  
  <p><code>Observable.of(a,b).merge()</code> is not possible because of type parameter difference.</p>