elements to identify (if only certain</span>
<span class="sd">            functional groups are of interest).</span>
<span class="sd">        :return: set of ints representing node indices</span>
<span class="sd">        &quot;&quot;&quot;</span>

        <span class="n">heteroatoms</span> <span class="o">=</span> <span class="nb">set</span><span class="p">()</span>

        <span class="k">for</span> <span class="n">node</span> <span class="ow">in</span> <span class="bp">self</span><span class="o">.</span>