kekrozsak/vendor/symfony/symfony/src/Symfony/Bundle/FrameworkBundle/Resources/views/Form/collection_widget.html.php

<?php if (isset($prototype)): ?>
    <?php $attr['data-prototype'] = $view->escape($view['form']->row($prototype)) ?>
<?php endif ?>
<?php echo $view['form']->widget($form, array('attr' => $attr)) ?>