* <li>weekly</li>
     * <li>monthly</li>
     * <li>yearly</li>
     * <li>never</li>
     * </ul>
     * The value "always" should be used to describe documents that change each
     * time they are accessed. The value "never" should be used to describe
     * archived URLs.
     *
     * Please note that the value of this tag is considered a hint and not a