an>
        <span class="k">return</span> <span class="nb">list</span><span class="p">(</span><span class="n">plist</span><span class="o">.</span><span class="n">keys</span><span class="p">())</span>

    <span class="nd">@staticmethod</span>
    <span class="k">def</span> <span class="nf">_check</span><span class="p">(</span><span class="n">i</span><span class="p">,</span> <span class="n">x</span><span class="p">):</span>
<span class="w">        </span><span class="sd">&quot;&quot;&quot;</span>
<span class