title: Put condition to andThen method of Completable
body: <p>I have a <code>Completable</code> created like this:</p>
  
  <pre><code>public Completable doCalulations() {
      return Completable.fromCallable(() -&gt; {
          //some calculations
      })
      .andThen(/*Here I want to sequentially execute another Completable*/);
  }
  </code></pre>
  
  <p>After first <code>Completable</code> calls <code>onComplete</code> I want to execute sequentially another <code>Completable</code> based on some condition:</p>
  
  <pre><code>if (condition.check()) {
      return someCalculation(); //returns Completable
  } else {
      return anotherCalculation(); //returns Completable
  }
  </code></pre>
  
  <p>How can I do this?</p>