span class="n">hdim</span><span class="p">,</span> <span class="n">hdim</span><span class="p">:</span><span class="n">dim</span><span class="p">]</span>
                <span class="n">x21</span> <span class="o">=</span> <span class="n">mat</span><span class="p">[</span><span class="n">hdim</span><span class="p">:</span><span class="n">dim</span><span class="p">,</span> <span class="mi">0</span><span class="p">:</span><span class="n">hdim</span><span class="p">]</span>
                <span class="n">x22</s