ChoicesWithPredefinedCollection

// within the controller it works like this:

$defaultData = ['books' => $booksVal];

       $myForm = $this->createFormBuilder($defaultData)
            ->add('books', ChoiceType::class, [
                 'choices' => $defaultData,
                'choice_label' => function ($value) { return $value; },

// however, integrated in a CustomType it doesn't:

$form = $this->createForm(
            /* type */ CustomeType::class,
            /* data */ ['books' =>  $booksVal]);

// here is the CustomType:

 public function buildForm(FormBuilderInterface $builder, array $options)
    {


        $builder
            ->add('books', ChoiceType::class, [
               'choices' => /* what I'm supposed to write here? */ ,
                'choice_label' => function ($value) { return $value; },
                'multiple' => true,
                'expanded' => true

// The Symfony docu says, but how do I get $group within my CustomType?
// @link: https://symfony.com/doc/master/reference/forms/types/entity.html ('Using Choices')

$builder->add('users', EntityType::class, array(
    'class' => 'AppBundle:User',
    'choices' => $group->getUsers(), // how can this be achieved?
));