title: Combining and filtering multiple events with WhenAnyObservable
body: <p>I have a ViewModel with:</p>
  
  <ul>
  <li>a list ReactiveList&lt; MyObject></li>
  <li>a single MyObject </li>
  <li>an IsBusy boolean</li>
  </ul>
  
  <p>I'd like to disable the command when none of the "Active" property from the MyObject list or from the single MyObject is true or in any case when IsBusy is true.</p>
  
  <p>Before adding the IsBusy to the picture the best solution I came up with was:</p>
  
  <pre><code>var canSave = this.WhenAnyObservable(
      x =&gt; x.MyObjectsList.ItemChanged, x =&gt; x.MyObject.Changed)
      .Where(x =&gt; x.PropertyName == "Active")
      .Select(_ =&gt; MyObjectsList.Any(x =&gt; x.Active) || MyObject.Active);
  
  SaveCommand = ReactiveCommand.Create(Save, canSave);
  </code></pre>
  
  <p>The idea is that it re-evaluates every time an Active property changes.
  Not sure whether this is the best solution at all (therefore any suggestion to improve it is welcome), but definitely I'm unable to add the IsBusy to the picture in order to re-evaluate the Select clause (including IsBusy state) when IsBusy changes.</p>