#(define (not-has-duration? music)
(not (ly:duration? (ly:music-property music 'duration))))

  
#(define (trim-list chord-elements n)
(cond 
	((or	(null? (cdr chord-elements))
			(not-has-duration? (car (cdr chord-elements))))
		chord-elements)	
	((<= n 1) 
		(cons	(car chord-elements)
				(filter not-has-duration? chord-elements)))
	(else			
		(trim-list (cdr chord-elements) (1- n)))
))

  
extractNote = #(define-music-function (parser location n music )(number? ly:music?)
(music-map	
  (lambda (evt)
  	(if (eq? 'EventChord (ly:music-property evt 'name))
		(ly:music-set-property! evt 'elements 
				(trim-list (ly:music-property evt 'elements) n)))
	 evt)			
  music))	
  
%%%%%%%%%%%%%%%%%%%%%%%  
#(define (get-element lst n)
(cond 
	((null? (cdr lst)) (car lst))
	((eq? 'VoiceSeparator (ly:music-property (car lst) 'name)	)
		(get-element (cdr lst) n))
	((<= n 1) (car lst))
	(else			
		(get-element (cdr lst) (1- n)))
))

  
extractVoice = #(define-music-function (parser location n music )(number? ly:music?)
(music-map	
  (lambda (evt)
  	(if (and
			(eq? 'SimultaneousMusic (ly:music-property evt 'name))
			(not (memq 'notExtractVoice  (ly:music-property evt 'tags))))	
		(make-music 'SequentialMusic 'elements (list
				(get-element (ly:music-property evt 'elements) n)))
		evt))			
  music))	

%%%%%%%%%%%%%%%%%%%%%%

extractPartUpper = #(define-music-function (parser location music )(ly:music?)
#{ 
	\extractNote #1000 \extractVoice #1 $music
#})

extractPartLower = #(define-music-function (parser location music )(ly:music?)
#{ 
	\extractNote #1 \extractVoice #1000 $music
#})

%% Décommenter ci dessous si vous voulez tester ....  
%{ 
music = \relative c'{
	<c e g>4-> <d f>( <b g' d'>) <c e g>-.
	<<
	{e4 f g2 a4 b c2}
	\\
	{ c,2 b4 d g,2 c2}
	>>
}

\score { <<
		\new Staff   \music
		\new Staff	\extractPartUpper \music 
		\new Staff	\extractPartLower\music 
	>>
} %}