transparent stems

[Jean-marc LEGRAND]

Hi list !

Working on Lily2.2 and XP, I cannont find how to translate the "\property 
Staff.Stem \set
#'transparent = ##t" I used on Lily2.0. I have a staff with 3 voices, and I 
want to make the stems
of the middle voice transparent.
I've tried  \override Staff.Stem #'transparent = ##t and it makes the wholes 
stems transparent (in
the 3 voices).
So I've tried \override Voice.Stem #'transparent = ##t, and nothing happens.
I've also tried the Lilybook trick : \override Stem #'transparent = ##t : idem.

What can I do, please ?

Here's my script :

basseI =    \notes
                  \context Staff <<
            \new Voice {\voiceOne
            \time 4/4
            \clef bass
                  \relative c {
f2 f
\bar "||"
}
}
            \new Voice  {\voiceTwo
            \relative c {
\stemDown
f,2 f
            }
            }

            \new Voice  {\voiceThree
            \relative c' {
\override Stem #'transparent = ##t
<<{a,2}\\{c}>> <<{a}\\{c}>>
            }
            }
>>