span><span class="n">x</span><span class="p">(</span><span class="n">idm</span><span class="p">))</span>
  <span class="n">dx</span><span class="p">(</span><span class="n">idm</span><span class="p">)</span> <span class="o">=</span> <span class="mf">0.5_rk</span><span class="o">*</span><span class="p">(</span><span class="n">x</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span><span class="o">-</span><span class="n">x</span><span class="p">(</span><span class="n">idm</span><span c