el</code> is provided, it is called in a similar way to <code>onListen</code>
when the returned stream stops having listener. If it later gets
a new listener, the <code>onListen</code> function is called again.</p>
<p>Use the callbacks, for example, for pausing the underlying subscription
while having no subscribers to prevent losing events, or canceling the
subscription when there are no listeners.</p>
    </section>
    
    <section class="summary source-code" id="source">
      <h2><span>Implementation<