        class="n">l</span>
        <span class="n">a_l</span><span class="p">[</span><span class="n">u</span><span class="p">]</span><span
            class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">v</span><span
            class="p">)</span>
        <span class="n">a_l</span><span class="p">[</span><span class="n">v</span><span class="p">]</span><span
            class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">u</span><span
    