trill spanner with "fancy" markup

[Vaclav Smilauer]

Hello,

I need to typeset a trill with the "tr" at the beginning and moreover 
being enclosed in parentheses, like this : "(tr~~~~~~~~~)".

I only need to concatenate two markups when setting edge-text, for the 
first argument of #cons; for simple "tr~~~" this is

\property Voice.TextSpanner \set #'edge-text = #(cons 
(make-musicglyph-markup "scripts-trill")  "").

So I tried

#(cons
   (combine-markup
       (make-musicglyph-markup "accidentals-leftparen")
       (make-musicglyph-markup "scripts-trill"))
   (make-musicglyph-markup "accidentals-rightparen"))

but this does not work (ERROR: In procedure car: ERROR: Wrong type 
argument in position 1: () ). I am not sure if combine-markup is the 
function I need, is there another one or should I do it in another way 
altogether?

Thanks for suggestions.

Vaclav

address@hidden:~/music/ly$ lilypond --version
lilypond (GNU LilyPond) 2.1.0