Vertical spacing

[address@hidden]

Neither the \override nor between-system-padding has any effect in the example 
below.
Sorry, I might have missed something, but found no solution. Could anyone 
please explain?

Thomas
LilyPond 2.13.4 on Debian 5.0

--
\version "2.13.4"
 
\paper {
 between-system-padding = 1.0 \in
 }

music = \relative c'
{
 \override Staff.VerticalAxisGroup #'minimum-Y-extent = #'( -20 . 0 )
 \repeat unfold 24 {  { c d e f } }
}

\score
{
 <<
  \new Staff \music
  \new Staff \music
 >>
}