class="identifier">point_xy</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="identifier">point</span><span class="special">;</span>

    <span class="identifier">bg</span><span class="special">::</span><span class="identifier">set</span><span class="special">&lt;</span><span class="number">0</span><span class="special">&gt;(</span><span class="identifier">point</span><span class="special">,</span> <span class="number">1</span><span clas