Manipulating grob properties doesn't work

[Marc Hohl]

I am starting to implement bends in lilypond.
As a first step, I want to make the right tab note head
covered by a slur transparent.

This worked fine with ties, but when I try the same with slurs,
it simply shows no difference.

What am I doing wrong here?

Marc

---------------------------------

\version "2.13.3"

test = \relative c {
 c4 ( d ) e ( f )
 c4( cis) d ( es )
 c4( es) e( gis )
 c4 ( cih ) c4 ( cisih )
}

#(define-public (slur::tab-bend grob)
 (let* ((original (ly:grob-original grob))
        (left-bound  (ly:spanner-bound grob LEFT))
        (right-bound (ly:spanner-bound grob RIGHT)))
        ;; original is not used at the moment, but there will be
        ,, different situations where an broken slur has to be handled 
differently

       (ly:grob-set-property! right-bound 'transparent #t)))
% this last line above doesn't seem to have any effect; why?

\layout {
 \context {
   \TabVoice
   \revert Slur #'stencil % I have tablature features installed and 
want the slurs to be visible for now
   \override Slur #'after-line-breaking = #slur::tab-bend
 }
}

\score {
  <<
     \new Staff { \clef "G_8" \test }
     \new TabStaff  { \clef "tab" \test }
  >>
}