title: Augment seperate values in observable of array - RxJS
body: <p>I've been trying to tackle this problem for a while now, I would guess this is a pretty common problem but what I want to do is augment items in an array coming from an observable, and the augmentation happens with an observable or promiselike
  So I want something like the following:</p>
  
  <pre><code>function augment(person: Person): Observable&lt;PersonWithAddress&gt; { 
      // does ajax call or something
  }
  
  const items$: Observable&lt;Person[]&gt;;
  
  items$
      .do(x =&gt; {}) // x would be of type Person[]
      .flatFlatMap(person =&gt; { // person would be of type Person
          return augment(person); // this would return an Observable&lt;PersonWithAddress&gt;
      })
      .subscribe(peopleWithAddresses =&gt; { // peopleWithAddresses would be of type PersonWithAddress[]
      })
  </code></pre>
  
  <p>Is there some kind of operator for this, I get I can augment or map a single item emitted from an observable to something else coming from an observable with flatMap, but is there something like flatFlatMap or so.</p>