      :param elements: List of elements that will qualify a carbon as special</span>
<span class="sd">            (if only certain functional groups are of interest).</span>
<span class="sd">            Default None.</span>
<span class="sd">        :return: set of ints representing node indices</span>
<span class="sd">        &quot;&quot;&quot;</span>

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

        <span class="c1"># For this fu