title: How to detect when an observable sequence hasn't produced a value after some time
body: <p>I have an observable that throttles <code>dragover</code> events by 350ms and returns <code>true</code> as long as those events keep coming in:</p>
  
  <pre><code>Rx.Observable
    .fromEvent(document, 'dragover')
    .map(function () { return true; })
    .throttle(350)
    .distinctUntilChanged()
  </code></pre>
  
  <p>I want to know if there hasn't been a dragover event withinin the last 350ms and return <code>false</code> when that happens. How should I go about doing that?</p>
  
  <p>Do I need to have a separate timer and reset it whenever a <code>dragover</code> event occurs? I'm hoping that I can keep this stateless.</p>