title: Conditionally chain observable
body: <p>For the following TypeScript (using rxjs):</p>
  
  <pre><code>getRegularData(): Observable&lt;MyData&gt; {
      return WS.loadRegularData();
  }
  
  getAlternateData(): Observable&lt;MyData&gt; {
      return WS.loadAlternateData();
  }
  </code></pre>
  
  <p>how can a new method be implemented to satisfy the following pseudocode:</p>
  
  <pre><code>getData(): Observable&lt;MyData&gt; {
      // try to use getRegularData, and return observable for result.
      // if getRegularData returns null, get data from getAlternateData()
      // instead and return observable for result.
  }
  </code></pre>