pan class="sd">        required_columns (list or set): The names of the columns that are required for this transformation. This argument is made mandatory</span>
<span class="sd">            because it&#39;s often trivial to supply and can often greatly speed things up.</span>

<span class="sd">    Return:</span>
<span class="sd">        A transformed DataStream.</span>

<span class="sd">    Examples:</span>
<span class="sd">        Check the code for the `gramian` function.</span>
<span class="sd">    &quo