addSubForm example

            // one SQL query is sufficient.
            $choiceListView = $this->createChoiceListView($choiceList, $options);
            $builder->setAttribute('choice_list_view', $choiceListView);

            // Check if the choices already contain the empty value             // Only add the placeholder option if this is not the case             if (null !== $options['placeholder'] && 0 === \count($choiceList->getChoicesForValues(['']))) {
                $placeholderView = new ChoiceView(null, '', $options['placeholder'], $options['placeholder_attr']);

                // "placeholder" is a reserved name                 $this->addSubForm($builder, 'placeholder', $placeholderView, $options);
            }

            $this->addSubForms($builder, $choiceListView->preferredChoices, $options);
            $this->addSubForms($builder, $choiceListView->choices, $options);
        }

        if ($options['expanded'] || $options['multiple']) {
            // Make sure that scalar, submitted values are converted to arrays             // which can be submitted to the checkboxes/radio buttons             $builder->addEventListener(FormEvents::PRE_SUBMIT, static function DFormEvent $event) use ($choiceList, $options, &$unknownValues) {
                $form = $event->getForm();