an><span class="p">[</span><span class="n">node</span><span class="p">])</span> <span class="ow">in</span> <span class="n">elements</span><span class="p">:</span>
                    <span class="n">heteroatoms</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">node</span><span class="p">)</span>
            <span class="k">else</span><span class="p">:</span>
                <span class="k">if</span> <span class="nb">str</span><span class="p">(</span><span class=