Vertical spacing between systems

[Dominik Baenninger]

I would like decrease the vertical space between systems of notes. If
found in the manual that I can manipulate this feature with

\set Staff.minimumVerticalExtent = #'(-4 . 4)

But in which way do I have to include this statement in the code?
Obviously it does not work in the way I tried it (see my code below).

Dominik

% ------------ my code --------------------
\include "hrnI.src"
\version "2.2.1"

% Set Font size
#(set-global-staff-size 18)

\score {
   << \hornOneNote>>
}

\paper{
   \set Staff.minimumVerticalExtent = #'(-4 . 4)
   #(set-paper-size "a4")
   linewidth = #(* mm 190)
}