public function subscriber_selection_form(user_selector_base $existinguc, user_selector_base $potentialuc) {
        $output = '';
        $formattributes = array();
        $formattributes['id'] = 'subscriberform';
        $formattributes['action'] = '';
        $formattributes['method'] = 'post';
        $output .= html_writer::start_tag('form', $formattributes);
        $output .= html_writer::empty_tag('input', array('type'=>'hidden', 'name'=>'sesskey', 'value'=>sesskey()));

        $existingcell = new html_table_cell();
        $existingcell->text = $existinguc->display(true);
        $existingcell->attributes['class'] = 'existing';
        $actioncell = new html_table_cell();
        $actioncell->text  = html_writer::start_tag('div', array());
        $actioncell->text .= html_writer::empty_tag('input', array('type'=>'submit', 'name'=>'subscribe', 'value'=>$this->page->theme->larrow.' '.get_string('add'), 'class'=>'actionbutton'));
        $actioncell->text .= html_writer::empty_tag('br', array());
        $actioncell->text .= html_writer::empty_tag('input', array('type'=>'submit', 'name'=>'unsubscribe', 'value'=>$this->page->theme->rarrow.' '.get_string('remove'), 'class'=>'actionbutton'));
        $actioncell->text .= html_writer::end_tag('div', array());
        $actioncell->attributes['class'] = 'actions';
        $potentialcell = new html_table_cell();
        $potentialcell->text = $potentialuc->display(true);
        $potentialcell->attributes['class'] = 'potential';

        $table = new html_table();
        $table->attributes['class'] = 'subscribertable boxaligncenter';
        $table->data = array(new html_table_row(array($existingcell, $actioncell, $potentialcell)));
        $output .= html_writer::table($table);

        $output .= html_writer::end_tag('form');
        return $output;
    }