an>
                        <span class="k">if</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">periods</span><span class="p">:</span>
                            <span class="n">periods</span><span class="p">[</span><span class="n">x</span><span class="p">]</span> <span class="o">+=</span> <span class="n">s</span>
                        <span class="k">else</span><span class="p">:</span>
                            <span class="n">periods</span><span class="p">[</span><span class