collision: system-separator and rehearsal mark

[Martin Tarenskeen]

In the following example the boxed rehearsal marks collide with the 
slashSeparator at the beginning of each system

%%% start of tiny example %%%

\version "2.16.0"

\paper {
  system-separator-markup = \slashSeparator
}

example = \relative c' {
  \set Score.markFormatter = #format-mark-box-alphabet
  \repeat unfold 4 {
    \mark \default {
      \repeat unfold 4 { c1 | }
      \break
    }
  }
}

\score {
  <<
    \new GrandStaff
    \new Staff \example
    \new Staff \example
    \new Staff \example
  >>
  \layout{}
}

%%% end of tiny example %%%