title: Getting the difference between 2 Observables and create 2 new observables from result
body: <p>I have 2 observables:</p>
  
  <pre><code>this.fb.getGroupsAsObservable().subscribe(
    (list: any) =&gt; {
      this.allGroups = list;
    }
  );
  
  this.fb.getContactsGroups(this.contact).subscribe(
    (list: any) =&gt; {
      this.groupsIn = list;
    }
  );
  </code></pre>
  
  <p>This works really well. But now I need to create a new Observable which is euqal to the difference between the 2 observables, which I will name 'groupsIn'. I am a bit new to rx/js, normally I would do a filter with .has() but this does not work on Observables.</p>