[plt-scheme] The filename of the file in the definition window

This is almost certainly a can't-see-the-tree-for-the-forrest problem.

I'm writing a small DrScheme tool and need the filename
(if there is one) in the front-end method.
But how do I get it?

   (define stylesheet-lang-mixin
     (class* object% (drscheme:language:language<%>)
       ...
       (define/private (front-end port settings)
           <need the filename here>
       )
       ...)

-- 
Jens Axel Søgaard