an> <span class="special">&amp;</span><span class="identifier">x</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">DATA_TYPE</span> <span class="special">&amp;</span><span class="identifier">y</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">{</span>
    <span class="keyword">if</span> <span class="special">(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">birth</span> <s