pan class="special">)</span>
<span class="special">&gt;::</span><span class="identifier">type</span>
<span class="identifier">invert</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span><span class="special">)</span>
<span class="special">{</span>
    <span class="keyword">return</span>
        <span class="comment">// Evaluate it with our actions</span>
        <span class="identifi