er this stream contains any elements.</p>
<p>Waits for the first element of this stream, then completes the returned
future with <code>true</code>.
If this stream ends without emitting any elements, the returned future is
completed with <code>false</code>.</p>
<p>If the first event is an error, the returned future is completed with that
error.</p>
<p>This operation listens to this stream, and a non-broadcast stream cannot
be reused after checking whether it is empty.</p>
        </section>
                <