an class="special">&amp;</span><span class="identifier">a2</span><span class="special">,</span> <span class="identifier">sum_type</span> <span class="special">&amp;</span><span class="identifier">s</span> <span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
<span class="special">};</span>

<span class="comment">//...</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span 