tr):</span>
<span class="sd">                The second vertex of the edge.</span>

<span class="sd">        &quot;&quot;&quot;</span>
        <span class="k">if</span> <span class="ow">not</span> <span class="bp">self</span><span class="o">.</span><span class="n">has_node</span><span class="p">(</span><span class="n">u</span><span class="p">)</span> <span class="ow">or</span> <span class="ow">not</span> <span class="bp">self</span><span class="o">.</span><span class="n">has_node</span><span class="p">(</sp