public function display_header() {
        echo '<th class="header ' . $this->get_classes() . '" scope="col">';
        $sortable = $this->is_sortable();
        $name = get_class($this);
        $title = $this->get_title();
        $tip = $this->get_title_tip();
        if (is_array($sortable)) {
            if ($title) {
                echo '<div class="title">' . $title . '</div>';
            }
            $links = array();
            foreach ($sortable as $subsort => $details) {
                $links[] = $this->make_sort_link($name . '-' . $subsort,
                        $details['title'], isset($details['tip']) ? $details['tip'] : '', !empty($details['reverse']));
            }
            echo '<div class="sorters">' . implode(' / ', $links) . '</div>';
        } else if ($sortable) {
            echo $this->make_sort_link($name, $title, $tip);
        } else {
            if ($tip) {
                echo '<span title="' . $tip . '">';
            }
            echo $title;
            if ($tip) {
                echo '</span>';
            }
        }
        echo "</th>\n";
    }