pan class="k">assert</span> <span class="n">col</span> <span class="ow">not</span> <span class="ow">in</span> <span class="bp">self</span><span class="o">.</span><span class="n">sorted</span><span class="p">,</span> <span class="s2">&quot;cannot drop a sort key!&quot;</span>
    <span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="n">actual_cols_to_drop</span><span class="p">)</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
        <spa