pan><span class="bp">self</span><span class="p">,</span> <span class="n">u</span><span class="p">,</span> <span class="n">v</span><span class="p">):</span>
<span class="w">        </span><span class="sd">&quot;&quot;&quot;</span>
<span class="sd">        Adds an edge between the vertices u and v if they exist.</span>

<span class="sd">        Parameters:</span>
<span class="sd">            u (str):</span>
<span class="sd">                The first vertex of the edge.</span>
<span class="sd">            v (s