n class="n">add</span><span class="p">(</span><span class="n">node</span><span class="p">)</span>

        <span class="c1"># Condition four: oxirane/aziridine/thiirane rings</span>
        <span class="n">rings</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">molgraph</span><span class="o">.</span><span class="n">find_rings</span><span class="p">()</span>
        <span class="n">rings_indices</span> <span class="o">=</span> <span class="p">[</span><span cl