>if</span> <span class="bp">self</span><span class="o">.</span><span class="n">_cell_inverse</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
            <span class="bp">self</span><span class="o">.</span><span class="n">_cell_inverse</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">linalg</span><span class="o">.</span><span class="n">inv</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><s