Understanding spacing

[Noeck]

Hi,

I try to get the spacing commands right and this looks strange to me:

\version "2.16.0"
\paper {
  system-system-spacing #'basic-distance = #123
  system-system-spacing #'stretchability = #123
  #(display system-system-spacing)
}

produces in the output:

((stretchability . 123) (basic-distance . 123) (basic-distance . 12)
(minimum-distance . 8) (padding . 1) (stretchability . 60))

Why basic-distance and stretchability are not overwritten by the new values?

Second question: Is there a function to change all 4 values without
typing the whole alist, only the values? Like:
  system-system-spacing = \foo #12 #8 #1 #60
That would be handy.

Third question: Is there a way to change spacing relative to the current
settings?
Like system-system-spacing #'basic-distance += #5 % note the +=

Cheers,
Joram