minimum distance between staves in StaffGroup

[Federico Bruni]

I'm using Lilypond 2.13.9 and 'm trying to use the commands suggested 
here:
http://lists.gnu.org/archive/html/lilypond-user/2009-10/msg00499.html

\layout {
  \context {
    \StaffGroup
    \override StaffGrouper #'between-system-spacing #'padding = #1.0
    \override StaffGrouper #'after-last-system-spacing #'padding = #1.0
  }
}

I want to set a minimum distance (or padding) between two staves.
I've tried to fiddle with values and properties but I can't get any 
result.
As the documentation on this topic is still missing (AFAIK), could you 
please give me a hint?

I get also a warning (maybe it's not that relevant):

warning: cannot find property type-check for 
`after-last-system-spacing' (backend-type?).  perhaps a typing error?
warning: doing assignment anyway

the same for between-system-spacing

Thanks for help,
Federico
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