vertical spacing of StaffGroup

[Werner LEMBERG]

What must I do to make

  \override VerticalAxisGroup #'minimum-Y-extent = #'(-10 . 10)

work for the `StaffGroup' context?  I want a score layout like this
(each horizontal line represents a staff):


      /
      |-----------|             \new StaffGroup = "sg1" <<
      |-----------|               \new Staff = "s11" {...}
      |-----------|               \new Staff = "s12" {...}
      \                           \new Staff = "s13" {...}
      |                         >>
      |
      /                         \new StaffGroup = "sg2" <<
      |-----------|               \new Staff = "s21" {...}
      |-----------|               ...
      |-----------|
      \
      |
      |
      /
      |-----------|
      |-----------|
      |-----------|
      \


I could fiddle with VerticalAxisGroup of single staff lines, but...
I could also insert invisible staves, but...


    Werner