tifier">g</span><span class="special">;</span> <span class="identifier">P</span> <span class="identifier">x</span> <span class="special">=</span> <span class="identifier">h</span><span class="special">;</span> <span class="comment">// The effect of 
</span><span class="identifier">add_intersection</span><span class="special">(</span><span class="identifier">s1</span><span class="special">,</span> <span class="identifier">y</span><span class="special">,</span> <span class="identifier">x</span><span class="sp