trnasparent stem function

[Jean-marc LEGRAND]

Hi list !

Using 2.9 on XP.

I'm beginning to engrave Marais' Viol suites (since I've understood eps files 
inclusion...).

I'm stuck with this : in the fac simile, inside notes of a chord are stemless. 
So I have to get
something like
<<{a4} \\ {<c e> % without stem} \\ {a}>>

So I've decided to get in function construction, and I've tried this for a 
simple piece of music :

\version "2.9"
stemLess = #(define-music-function (parser location note) (ly:music?)
       #{\once \override Voice.Stem #'transparent = ##t       #})

stemFull = #(define-music-function (parser location note) (ly:music?)
       #{\revert  Voice.Stem #'transparent  #})

\score {
      \relative c'' {
            c4 \stemLess c c \stemFull c
            }
      }

I get
-  the first c with a stem (ok)
- the second one ithout stem (ok)
- no more music after that : not OK at all.

I think I'm making a mistake with my function, since it's the first aof my 
entire life !

Does anyone know where it is ?

Best regards !

JMarc