 class="identifier">sum_type</span><span class="special">[</span> <span class="identifier">sum_size</span> <span class="special">];</span>

    <span class="keyword">void</span>  <span class="keyword">operator</span> <span class="special">()(</span> <span class="identifier">addend1_type</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="identifier">a1</span><span class="special">,</span> <span class="identifier">addend2_type</span> <span class="keyword">const</span> <sp