 class="w">         </span><span class="n">nm1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">N</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">xprev</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">X</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="w">         </span><sp