special">}</span>
        <span class="keyword">throw</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">runtime_error</span><span class="special">(</span><span class="string">"Unsupported filter relation: "</span> <span class="special">+</span> <span class="identifier">rel</span><span class="special">);</span>
    <span class="special">}</span>
<span class="special">};</span>

<span class="keyword">void</span> <span class="identifier">init_factories</span><spa