title: Rx filter by criterion and n values after criterion
body: <h3>Filter by criterion only</h3>
  <pre><code>changes.Where(p =&gt; Evaluate(p)).Subscribe(p =&gt; { // Do something });
  </code></pre>
  <p><em>But how could you make it so that you get the criterion value and n values after the criterion is fulfilled (and these n values do not have to match the evaluation criterion)?</em></p>
  <ul>
  <li>e.g. i would like to subscribe to a stream that returns on <code>Evaluate(p)</code> and one value after that (and then starts to evaluate <code>p</code> again)</li>
  </ul>