dtU</span><span class="p">[</span><span class="n">dtU</span><span class="o">&lt;</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="n">dtU</span><span class="p">[</span><span class="n">dtU</span><span class="o">&lt;</span><span class="mi">0</span><span class="p">]</span> <span class="o">+</span> <span class="mi">2</span><span class="o">*</span><span class="n">np</span><span class="o">.</span><span class="n">pi</span>
        <span class="n">dtL</span><span class="p