title: Multiple subscriptions nested into one subscription
body: <p>I find myself puzzled trying to set a very simple rxjs flow of subscriptions. Having multiple non-related subscriptions nested into another.</p>
  
  <p>I'm in an angular application and I need a subject to be filled with next before doing other subscriptions.</p>
  
  <p>Here would be the nested version of what I want to achieve.</p>
  
  <pre><code>subject0.subscribe(a =&gt; {
  
      this.a = a;
  
      subject1.subscribe(x =&gt; {
          // Do some stuff that require this.a to exists
      });
  
      subject2.subscribe(y =&gt; {
          // Do some stuff that require this.a to exists
      });
  
  });
  </code></pre>
  
  <p>I know that nested subscriptions are not good practice, I tried using <code>flatMap</code> or <code>concatMap</code> but didn't really get how to realize this.</p>