using variable within make-pango-font-tree

[menelikke]

Hi

does anybody knows if and how is possibile to use a variable within
"make-pango-font-tree" function, please?

I'm using

\version "2.18.2"  % necessary for upgrading to future LilyPond versions.
\include "global.ly"
\paper {
  #(define fonts
     (make-pango-font-tree  \docfont 
                            "Arial"                                   
                            "Arial"
                            (/ staff-height pt 25)
     )
   )
.
.

where docfont is a variable defined as

docfont = "Papyrus"

in the external "global.ly" file.

Using that i get a "GUILE" error at expression starting from here
  #
   (define fonts

Please note that 
docfont = "\"Papyrus\""
doesn't work too. 

Thanks