 jsut in case &#39;S&#39; is called</span>

            <span class="c1"># calculate amount of white, W:</span>
            <span class="n">W</span> <span class="o">=</span> <span class="mi">1</span> <span class="o">/</span> <span class="p">(</span><span class="mf">1.0</span> <span class="o">+</span> <span class="n">cW</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="o">*</span><span class="p">(</span><span class="n">s</span><span class="o">**</span><span class="n"