Usage of ly:stencil-fonts ??

[Thomas Morley]

Hi,

in the IR one can find:
"
Function: ly:stencil-fonts s
Analyze s, and return a list of fonts used in s.
"

Though, how to get some output from it?

At least the code below returns nothing relevant.
(I used old syntax to check with all lily-version available, i.e.
since 2.12.3 up to 2.21.0)

#(use-modules (ice-9 pretty-print))

{
  \override NoteHead #'after-line-breaking =
  #(lambda (grob)
    ;(pretty-print (ly:stencil-expr (ly:grob-property grob 'stencil)))
    (write (ly:stencil-fonts (ly:grob-property grob 'stencil))))

  c'1

  \override TextScript #'after-line-breaking =
  #(lambda (grob)
    ;(pretty-print (ly:stencil-expr (ly:grob-property grob 'stencil)))
    (write (ly:stencil-fonts (ly:grob-property grob 'stencil))))
  s-\markup \number "1"
}

Something wrong with my code? Or stopped it working long time ago? Did
it ever work?
(I found not a single usage)

Afaict, ly:stencil-fonts was last changed with

 commit 7a6c3f44dad456a55a4d67c8647332d16022e7d1
 Author: Han-Wen Nienhuys <address@hidden>
 Date:   Fri Jan 7 12:40:11 2005 +0000

     *** empty log message ***

Any hints?

Cheers,
  Harm