span><span class="n">cb</span>

            <span class="c1"># create Aab:</span>
            <span class="n">Aab</span> <span class="o">=</span> <span class="n">ajoin</span><span class="p">((</span><span class="n">A</span><span class="p">,</span><span class="n">a</span><span class="p">,</span><span class="n">b</span><span class="p">))</span>    

            <span class="c1"># calculate rgbc:</span>
            <span class="n">rgbc</span> <span class="o">=</span> <span class="n">np</span><span class="o">.<