Beam settings

[Maurizio Tomasi]

I have a problem with beams in Lilypond 1.5.59.  Consider this file:

----------------------------------------------------------------------
\score {
    \notes \relative g'' {
        \time 6/8

        g8 g, g' a a, a' | b b, b' c c, c'
    }
}
----------------------------------------------------------------------

Lilypond 1.5.60 creates a good-looking score, but Lilypond 1.5.59 (which
is installed on my home computer) does not draw very well the first two
beams [g8 g, g'] and [a a, a'].  I tried to change `staff-position' in
Voice.Beam:

----------------------------------------------------------------------
\property Voice.Beam \set #'staff-position = #-10
----------------------------------------------------------------------

But this line has no effect!  I can put -2, -4, -10 or whatever else, and
the beams are still the same.  Am I using the wrong property?  In fact, I
looked on a old Lilypond 1.4.7 manual, since I am not able to compile the
1.5.x documentation.

Thanks
Maurizio