mple from the section
            <a href="customizing.htm#extending_the_list"><i>Extending the list</i></a>
            in the <i>Customizing Guide</i>, with the
            differences highlighted.
        </p>
        <table>
            <tbody>
            <tr class="first_row">
                <td>2.7</td>
                <td>3.0</td>
            </tr>
            <tr>
                <td><pre class="tty">def <span class="changed">get_extension</span>(self, timespan, <span
                        class