Trying to reduce space between individual systems

[Carl Youngblood]

I found the following tip on page 242 of the lilypond manual:

\overrideProperty
#"Score.NonMusicalPaperColumn"
#'line-break-system-details
#'((fixed-alignment-extra-space . 5))

for increasing the space between individual systems.  What about the
opposite?  Is there an easy way for me to reduce the space between
individual systems in my piece?

Thanks,
Carl