title: RxJava. Zip operator: sequence contains no elements exception
body: <p>Code throws <code>Sequence contains no elements</code> exception. How to fix it?</p>
  
  <pre><code>Observable&lt;Entity&gt; observable = observable.fromCallable(…);
  Observable&lt;Entity&gt; subObservable1 = observable.flatMap(…)
  Observable&lt;Entity&gt; subObservable2 = observable.flatMap(…)
  Observable&lt;OtherEntity&gt; subObservable3 = subObservable2.reduce(…); 
  Observable&lt;List&lt;AnotherEntity&gt;&gt; subObservable4 = subObservable3.map(…);
  
  return Observable.zip(
      observable, subObservable1, subObservable2, 
      subObservable3, subObservable4) -&gt; … );
  </code></pre>
  
  <p>I suppose problem is related to the Zip operator because of <code>observable</code> is used 3 times (in <code>zip(...)</code>, in <code>subObservable1</code>, in <code>subObservable2</code>);</p>
  
  <p>I tried to share first <code>observable</code>, also tried to publish it, but it didn't solve the issue.</p>