function add_columns()
 {
     $screen = get_current_screen();
     if ('edit' != $screen->base) {
         return;
     }
     $post_type = $screen->post_type;
     foreach (self::$column_args as $p2p_type => $column_args) {
         $ctype = p2p_type($p2p_type);
         $directed = $ctype->find_direction($post_type);
         if (!$directed) {
             continue;
         }
         if (!('any' == $column_args || $directed->get_direction() == $column_args)) {
             continue;
         }
         $column = new P2P_Column($directed);
         $column->styles();
         add_filter("manage_{$screen->id}_columns", array($column, 'add_column'));
         add_action("manage_{$post_type}_posts_custom_column", array($column, 'display_column'), 10, 2);
     }
 }
 function display_column($column, $item_id)
 {
     echo parent::render_column($column, $item_id);
 }
 function display_column($content, $column, $item_id)
 {
     return $content . parent::render_column($column, $item_id);
 }