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accessing accidentals with \tweak

From: Mark Polesky
Subject: accessing accidentals with \tweak
Date: Tue, 14 Jul 2009 13:15:55 -0700 (PDT)

In NR 5.3.4 "The \tweak command", it says this:

Notably the \tweak command cannot be used to modify stems, beams
or accidentals, since these are generated later by note heads,
rather than by music elements in the input stream.

But I *am* able to abuse the notehead's stencil property to tweak
the accidental:

\version "2.13.2"

#(define (color-accidental grob)
  ;; notehead stencil callback
  (let ((stil       (ly:note-head::print grob))
        (accidental (ly:grob-object grob 'accidental-grob)))
    (ly:grob-set-property! accidental 'color red)
#(define (suppress-accidental grob)
  ;; notehead stencil callback
  (let ((stil       (ly:note-head::print grob))
        (accidental (ly:grob-object grob 'accidental-grob)))
    (ly:grob-suicide! accidental)

\relative { 
  <c! \tweak #'stencil #color-accidental e! g!>
  <c! \tweak #'stencil #suppress-accidental e! g!>


1) are the results of this technique undefined? Was it just luck
   that these worked?
2) This seems so kludgy. I presume I can only access the
   NoteHead's 'accidental-grob property through its standard
   settings (duration-log, stem-attachment, stencil, X-offset, and
   Y-offset). I wish I could do something like this instead (this
   of course doesn't work) ---
   #(define (color-accidental grob)
     ;; notehead foo callback
     (let ((accidental (ly:grob-object grob 'accidental-grob)))
       (ly:grob-set-property! accidental 'color red)

   \relative { 
     <c! \tweak #'foo #color-accidental e! g!>

   This would imply that I could make up my own dummy property and
   it would (magically) have access to the note-head-interface
   internal properties. Then I wouldn't have to bother making sure
   I return the correct value for whatever standard setting I'm
   abusing (stencil in the above case). Besides, I'd want to be
   able to tweak that standard setting independently. As a
   compromise, I considered the possibility of actually
   implementing a 'dummy property, but I don't think that would
   work either, because each subsequent call to \tweak #'dummy
   would override the previous one (for a given note).
This bizarre approach might solve a lot of problems for me, but I
don't like abusing properties like this. What do you guys think?
- Mark


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