public function format_expander($field, $column)
    {
        if (!$this->current_row[$field]) {
            $this->current_row[$field] = $column['descr'];
        }

        // TODO:
        // reformat this using Button, once we have more advanced system to
        // bypass rendering of sub-elements.
        // $this->current_row[$field] = $this->add('Button',null,false)
        $key = $this->name.'_'.$field.'_';
        $id = $key.$this->app->normalizeName($this->model->id);
        $class = $key.'expander';

        $this->current_row_html[$field] =
            '<input type="checkbox" '.
                'class="'.$class.'" '.
                'id="'.$id.'" '.
                'rel="'.$this->app->url(
                    $column['page'] ?: './'.$field,
                    array(
                        'expander' => $field,
                        'expanded' => $this->name,
                        'cut_page' => 1,
                        // TODO: id is obsolete
                        //'id' => $this->model->id,
                        $this->columns[$field]['refid'].'_'.$this->model->id_field => $this->model->id,
                    )
                ).'" '.
            '/>'.
            '<label for="'.$id.'" style="cursor:pointer;"><a class="atk-button-small">'.
            $this->current_row[$field].'</a></label>';
    }