>if</span> <span class="ow">not</span> <span class="n">E</span><span class="p">:</span>
            <span class="n">E</span> <span class="o">=</span> <span class="p">[]</span>
        <span class="n">N</span><span class="o">.</span><span class="n">append</span><span class="p">(((</span><span
            class="n">x_parent</span><span class="p">,</span> <span class="n">y_parent</span><span
            class="p">),</span> <span class="p">(</span><span class="n">node</span><span class="o">.</span><span
       