StaffGrouper vs. VerticalAxisGroup: syntax inconsistencies

[Werner LEMBERG]

Folks,


why can I say

  \new PianoStaff \with {
    \override StaffGrouper
      #'between-staff-spacing #'minimum-distance = #20
  } ...

but not

  \new Staff \with {
    \override VerticalAxisGroup
      #'next-staff-spacing #'minimum-distance = #20
  } ...

?

The latter gives this warning:

  warning: type check for `next-staff-spacing' failed;
  value `((minimum-distance . 12) .
          #<primitive-procedure 
ly:axis-group-interface::calc-next-staff-spacing>)'
    must be of type `list'

I would like to use former syntax form for the latter too, and it
seems to be a real syntax inconsistency since both the
`between-staff-spacing' and `next-staff-spacing' lists contain the
same elements.


     Werner