protected function buildNestedForm($column, \Closure $builder, $key = null)
    {
        $form = new Form\NestedForm($column, $key);

        $form->setForm($this->form);

        call_user_func($builder, $form);

        $form->hidden($this->getKeyName());

        $form->hidden(NestedForm::REMOVE_FLAG_NAME)->default(0)->addElementClass(NestedForm::REMOVE_FLAG_CLASS);

        return $form;
    }