RxSwift merge different kind of Observables
<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>