With a combination of Googling and reading the docs, I've managed get 
a form with an embedded 1:M collection up and running, but now I'm 
stumped.  What I want to do is theme a single field in an _embedded_ 
form. 

I've successfully themed single fields on the parent form, but what I 
expected to work on an embedded form isn't panning out. 
Say I have a parent form of type Company and a collection of embedded 
forms of type Facility which are inserted into the dom via JS using 
the collection type's form prototype. 

Say I want to customize just the 'name' field in the parent, Company 
form, as demonstrated here: 
http://symfony.com/doc/current/cookbook/form/form_customization.html#... 

If the ID is company_name I create a twig block with a name of 
_company_name_widget and it works perfectly. 

Now say I want to customize only the address field of each Facility in 
the collection.  The ID of that field in the prototype is 
company_facilities_$$name$$_address, and my JS would replace the $$name 
$$ with an index.  I've tried creating twig blocks with names with 
different combinations of 'companies', 'facilities', 'address' and 
underscores to see if any would register and so far no dice. 
Theoretically, if it follows the pattern, a twig block name of 
_company_facilities_$$name$$_address_widget should work, but twig 
isn't a fan of '$' in block names. 

So is this possible?