Barline spacing at rehearsal mark

[Nick Payne]

I want some extra spacing before and after the barlines where I have
rehearsal marks. I tried \override Score.RehearsalMark #'extra-spacing-width
(which made no visible change) and \override Staff.BarLine
#'extra-spacing-width (which only adds space after the barline, though the
documentation on extra-spacing-width indicates that it should add spacing on
both the left and right side).

What command should I use?

\version "2.12.2"

\relative c'' {
c4 c c c 
\once \override Score.RehearsalMark #'extra-spacing-width = #'(2 . 2)
\mark \markup { \box "A" }
c4 c c c
c4 c c c
\once \override Staff.BarLine #'extra-spacing-width = #'(2 . 2)
\mark \markup { \box "B" } 
c4 c c c }

BTW, trying to search the LSR for either extra-spacing-width or
"extra-spacing-width" just returns a syntax error.

Nick