</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="mi">1</span>
<span class="w">            </span><span class="n">del</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Z</span><span class="p">(</span><span class="n">ip1</span><span class="p">)</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="n">Z</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
<span c