Put the property of a grob in the output-attributes list

[Paolo Prete]

Hello all.

the following function (thanks to Stefano!) inserts the beam-thickness
property of a Beam in the list of output-attributes of the corresponding
SVG item. It does its job if I place the function soon before a beam symbol
"[".  But it doesn't work (no errors, but the output-attributes property is
not set) if  place it before a notename. How can I fix that? Should I
map-some-music() and iterate until I find the BeamEvent? I tried that too,
but without success.
Thanks.

token = #(let* ((ctr 0)
                (ctr! (lambda ()
                        (set! ctr (1+ ctr))
                        ctr)))
   (define-music-function (mus) (ly:music?)
     (let* ((id (format #f "foobar_~a" (ctr!)))
            (mexp #{ \tweak output-attributes.id #id #mus #} )
            (type (ly:music-property mus 'name))
            (mexp (case type
                    ('BeamEvent
                      #{ \tweak Beam.after-line-breaking #(lambda (grob)
                           (let* ((outprop (ly:grob-property grob
'output-attributes))
                                  (beam-thickness (ly:grob-property grob
'beam-thickness))
                                  (outprop (append outprop
`((beam-thickness . ,beam-thickness)))))
                             (begin
                                  (ly:grob-set-property! grob
'output-attributes outprop)
                                  (display "\n****\n")
                                  (display beam-thickness)
                                  (display "\n****\n"))))
                      #mexp #} )
                    (else mexp))))
       mexp)))

\relative { c'4 d8-\token [ e ] \token f[ g ] c,4 }