an class="special">,</span> <span class="identifier">et_off</span><span class="special">&gt;&amp;</span> <span class="identifier">b</span><span class="special">)</span>
<span class="special">{</span>
    <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">a</span> <span class="special">+=</span> <span class="identifier">b</span><span class="special">);</span>
<span cla