ecial">);</span>       <span class="comment">// add_intersection 
</span><span class="identifier">s2</span> <span class="special">+=</span> <span class="special">(</span><span class="identifier">y</span> <span class="special">&amp;</span> <span class="identifier">x</span><span class="special">);</span>                    <span class="comment">// and &amp; followed by +=
</span><span class="identifier">assert</span><span class="special">(</span><span class="identifier">s1</span><span class="special">==</span