h>required?</th>
                <th>default</th>
                <th>choices</th>
            </tr>
            <tr>
                <td><code>key</code></td>
                <td>Key</td>
                <td>function</td>
                <td>No</td>
                <td></td>
                <td>
                </td>
            </tr>
            <tr>
                <td><code>src</code></td>
                <td>Source sequence</td>
                <td>expression</td>
                <td>Yes</td>
         