title: Take events only after an Observable from a chain has emitted one
body: <p>Here is the chain: <code>observableOne.flatMap(event -&gt; observableTwo).subscribe()</code></p>
  
  <p>I'd like to take events from <code>observableOne</code> the very first time it emits one, then ignore all other events from this <code>Observable</code> until after  <code>observableTwo</code> has either emitted a value, completed or finished with an error. Worth mentioning that ultimately I'm interested in the events from the second Observable.</p>
  
  <p>The context is like, there is a <code>Button</code> clicking on which an event is fired, this is <code>ObservableOne</code>. The fired event triggers <code>ObservableTwo</code>, let's say it's a chain of network operations. So I want to ignore all button clicks while network operations are being executed.</p>