RemoveEmptyStaves and the ambitus_engraver

[Roman Stawski]

> I'm not top posting.

It seems that \RemoveEmptyStaves doesn't play very well with the
ambitus_engraver.

In this example, I'm trying to get the treble staff removed in the
first two systems, and appear with an ambitus on the third. 

It is removed correctly in the first system. But reappears in the
second. If I comment out the line (*), then the staves are removed as
needed, but the ambitus is too of course. 

---
\version "2.17.11"
\paper { ragged-right = ##t }

\layout {
  \context {
    \type "Engraver_group"
    \name MainSequence
    \accepts Voice
    \consists Ambitus_engraver %% (*)

    \override AmbitusNoteHead.break-visibility = ##(#f #t #t)
    \override AmbitusLine.break-visibility = ##(#f #t #t)
    \override AmbitusAccidental.break-visibility = ##(#f #t #t)
  }
  \context {
    \Staff
    \accepts MainSequence
    \override VerticalAxisGroup.remove-empty = ##t
    \override VerticalAxisGroup.remove-first = ##t
  }
}

\new ChoirStaff <<

  \new Staff \relative c' {
    \clef treble
    \skip 1*8
    \new MainSequence {
      \repeat unfold 2 { e1 e'1 }
    }
    \bar "|."
  }

  \new Staff \relative c {
    \clef bass
    \repeat unfold 4 {e1} \break
    \repeat unfold 4 {e1} \break
    \repeat unfold 4 {e1}
  }
>>
---


It looks as if the ambitus is keeping the system alive. I can't get it
to work by setting keepAliveInterfaces in the staff context either.
Has anyone any ideas?

Thanks for any help

Roman