Difficult tweaks - not working!

[Vivian Barty-Taylor]

I'm having trouble with the example described 
in the "difficult tweaks" section of the manual.
I'm wanting a stem with two different noteheads 
on it. I started by copying the code from the manual 
(correcting the 'curly' ' symbols this time!) then 
changed various parameters. My code now looks like 
this:

#(define ((set-position-font-size pos style) 
grob origin current) 
(let* 
        ((interfaces 
                (ly:grob-property grob 'interfaces))
                (position (ly:grob-property grob 'staff-position))) 
        (if (and 
                ; is this a note head? (memq 'note-head-interface interfaces)
                ; is the Y coordinate right? 
                (= pos position)) 

                ; then do it. (set! (ly:grob-property grob 'style) style)))) 

\relative { 
        c 
        \applyoutput #(set-position-font-size -2 #'cross) 
<c e g> }

When I try to compile it I get the error output:

GUILE signaled an error for the expression beginning here
#
  (define ((set-position-font-size pos style) grob origin current) 
/Users/anneke/Desktop/Vivian/Lilypond/Noteheads1.ly:1:2: 
error: syntax error, unexpected '('
# 
  (define ((set-position-font-size pos style) grob 
origin current) end of file in /Users/anneke/Desktop/
Vivian/Lilypond/Noteheads1.ly
/Users/anneke/Desktop/Vivian/Lilypond/Noteheads1.ly:18:1: 
In expression (set-position-font-size -2 
cross):
/Users/anneke/Desktop/Vivian/Lilypond/Noteheads1.ly:18:1: 
Unbound variable: set-position-font-
size

What is wrong?!

Many thanks,
Vivian Barty-Taylor,
Netherlands.