title: create observable<bool> from observablecollection
body: <p>I have ObservableCollection&lt;T&gt; and I need to create observable&lt;bool&gt; which returns true if collection contains any elements</p>
  
  <p>I try to do this  </p>
  
  <pre><code>var collectionHasElementsObservable =
              Observable.FromEventPattern&lt;NotifyCollectionChangedEventHandler,NotifyCollectionChangedEventArgs&gt;(
                  ev =&gt; ((ObservableCollection&lt;MyType&gt;)_items).CollectionChanged += ev,
                  ev =&gt; ((ObservableCollection&lt;MyType&gt;)_items).CollectionChanged -= ev);
  </code></pre>
  
  <p>But I don't know how to convert this into IObservable&lt;bool&gt;</p>
  
  <p>How can I create observable&lt;bool&gt; from this?</p>