How to change the stem-length?

[harm6]

Hi,

using a beam-stencil-override, I'd wish to adjust the stem-length within the
beam-stencil-override. The following (boiled down) function works fine in
most cases, even with kneed-beams. But when using it in a cross-staff
example with kneed-beams, I've noticed a problem: Some stems are affected
some not. Oddly enough, when changing the global-staff-size, I retrieve
different results.


\version "2.15.13"

#(set-global-staff-size 20)

#(define ((stem-change y) grob)
   (ly:beam::print grob) 
      (begin
        ;; Changing the beam-color is a dummy for the original, very large
override.
        (ly:grob-set-property! grob 'color red)
        (map 
          (lambda (n)
            (ly:grob-set-property! n 'length y))
              (ly:grob-array->list (ly:grob-object grob 'stems))))
   (ly:beam::print grob))

%---- Test

top = \change Staff = "1"
bottom = \change Staff = "2"

music = \relative c {
        
        \override Beam #'auto-knee-gap = #0

        \override Beam #'stencil = #(stem-change 20)

                \bottom c32[ g' \top eis'']
                c[ b \bottom  g,]
                \top e''[ \bottom c,, \top  g'']
                \bottom c,,64[ \top g'' e' \bottom c,, \top  c'']
                f,,32 [f''' f,,, f''']
}

\score {
  \new PianoStaff <<
    \new Staff = "1" {
      s4 s8 s16
    }
    \new Staff = "2" {
      \clef bass
      \music
    }
  >>
}


What am I missing?

Best,
  Harm

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