Scoping problem in define-music-function

[Marc Hohl]

Hello list,

thanks to Reinhold Kainhofer and Wilbert Berendsen I managed to get a 
callback with an
additional argument.

Now I want to write a music function like this:

myFunction = #(define-music-function parser location fret music) 
(string? ly:music?)
 (let (( ... some expressions ... ))

 #{
    \override TabNoteHead #'stencil = #(my-special-callback fret)
 #}
 (..some more scheme stuff working on music...)
 #{
    \revert TabNoteHead #'stencil
 #}
))

but lilypond complains with

<string>:2:46: Fehler: GUILE signalisierte einen Fehler für den hier 
beginnenden Ausdruck
   \override TabNoteHead #(quote stencil) = #
                                             (my-special-callback fret)
Unbound variable: fret

So fret is not known within #{ ... #}.

Is there a way to define the function in an appropriate way? It doesn't 
matter if
the given variable fret is an argument to the music function or 
calculated within the let-block.

Thanks in advance

Marc