/span> <span class="n">y</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
        <span class="sd">&quot;&quot;&quot;Creates the kernel matrix based on the given lists of local</span>
<span class="sd">        features x and y.</span>

<span class="sd">        Args:</span>
<span class="sd">            x(iterable): A list of local feature arrays for each structure.</span>
<span class="sd">            y(iterable): An optional second list of features. If not specified</span>
<