I have found some problems on this documentationhttp://symfony.com/doc/master/cookbook/form/dynamic_form_modification.html#dynamic-generation-for-submitted-forms

When it's going to explain the ajax, it says that: (nearly the end of the article)
".. include:: /cookbook/form/dynamic_form_modification_ajax_js.rst.inc" but it doesn't show the code.

In 2.3 version its fine but I'm new in symfony and I don't know what it's the fix of that for the current version.