n class="identifier">push_back</span><span class="special">(</span><span class="identifier">xy</span><span class="special">(</span><span class="number">3</span><span class="special">,</span> <span class="number">1</span><span class="special">));</span>
    <span class="identifier">line</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">xy</span><span class="special">(</span><span class="number">4</span><span class="special">,</