font selection

[Gerard McConnell]

Hello,
A Microsoft security update prevents me from using Lilypond's  OpenType Century Schoolbook L.
I need to use an alternative for Lyrics and Tab numbers.  
I replaced the default fonts with the following from section 1.8.3 in the Notation Reference:

    myStaffSize=#20
    #(define fonts
    (make-pango-font-tree "Times New Roman"
                "Nimbus Sans"
                        "Luxi Mono"
          ( / myStaffSize 20 )             ))

Which is great.  However, if I remove myStaffSize=#20  and  ( / myStaffSize 20 )
it gets the error message:

C:/Documents and Settings/Dad/Desktop/musicPages/guitarTwinkle.ly:17:9: error: GUILE signaled an error for the _expression_ beginning here
    #
         (define fonts

Is there a simple answer for why the extra lines allow it to work?

Thanks,
Gerard