MMRs + system breaks + keepAliveInterfaces

[Saul Tobin]

Setting keepAliveInterfaces at the beginning or end of a system with an MMR
immediately preceding or following "bleeds over" into the wrong system if
rests were considered "worth living" on either side.

\layout {
  \context {
    \Score
    keepAliveInterfaces = #'(stem-interface
                             note-head-interface
                             rest-interface
                             )
  }
}

% Ex 1

\new Staff \with {
  \override VerticalAxisGroup.remove-empty = ##t
  \override VerticalAxisGroup.remove-first = ##t
} {
  R1
  \set Staff.keepAliveInterfaces = #'()
  \break
  c'1
}

% Ex 2

\new Staff \with {
  \override VerticalAxisGroup.remove-empty = ##t
  \override VerticalAxisGroup.remove-first = ##t
} {
  \set Staff.keepAliveInterfaces = #'()
  c'1
  \unset Staff.keepAliveInterfaces
  \break
  R1
}