Store values in a scheme variable to use after

[josé henrique padovani]

Hi... I'm trying to store a NoteHead's position (by using scheme 
functions) to be abble to use this value after (as a string in a markup)...
To do that I define a variable (myvar with the string "init") and then I 
use tweak to set! myvar to its new value...

But when I call myvar from a markup, it still finds the old argument 
"init"...

Why?
Is it possible to do something like what I am trying to do in another way?

Thank you,
josé

code:
\version "2.13.22"

#(define myvar "init")

#(define (notehead-get-staffposition notehead)
   ;; notehead is grob
   (ly:grob-property notehead 'staff-position))

#(define (change-myvar notehead)
   ;; notehead is grob
   (let ((sp (ly:number->string (notehead-get-staffposition notehead))))
     (format #t "~2&~a\n" (make-string 40 #\-))
    (begin
     (format #t "~a\n" sp)
     (set! myvar sp))))

\relative c' {
  c1^\markup{\text \myvar}

  \once \override NoteHead #'before-line-breaking = #change-myvar
  c
  g^\markup{\text \myvar}
  a^\markup{\text \myvar}
}

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