class="comment">// calculate polygon bounding box</span>
        <span class="identifier">box</span> <span class="identifier">b</span> <span class="special">=</span> <span class="identifier">bg</span><span class="special">::</span><span class="identifier">return_envelope</span><span class="special">&lt;</span><span class="identifier">box</span><span class="special">&gt;(*</span><span class="identifier">p</span><span class="special">);</span>
        <span class="comment">// insert new value</span>
        <