removing identical staves

[Helge Kruse]

I there, I am typesetting a piece for a small orchestra. There ares 
voice for the violas and cellos too. These violas play unisono for the 
first five pages. Then suddenly they play very different voices. I am 
reluctant to add a Viola I staff and a viola II staff to the whole 
score. The composer was it too. But at page six I need a new staff group 
for the violas.

I've have tried this with the RemoveEmptyStaves engraver similar like 
this (please ignore the wrong clef for the viola):

\version "2.16.0"
\include "deutsch.ly"

\score {
<<
  \new Staff \relative c'' { \repeat unfold 29 { c8 c c c c c c c } }
  \new StaffGroup <<
    \new Staff \relative c'' { c1 c c c c c c c c c c c c c c g8 a h c 
d c h a }
    \new Staff \relative c'' { R1 R R R R R R R R R R R R R R c8 h a g 
f g h c }
>>
>>
  \layout {
    \context {
      \Staff \RemoveEmptyStaves   }
  }
}

This approach has two drawbacks.
- First I have a staff line with rests at the beginning, where both 
viola players should play.
- Second I have a measure #15 at the line where they play different 
melodies. The actual first measure of the line where the different 
melodies start can not be predicted easily.

I could try to quote the first viola voice in the second to get the 
correct notes in the first line. But I would prefer to write only one 
viola line at first page. Further the count of measures could change 
when I change the paper format.

I could also add a \break before measure #15 to control the lines 
breaks. But I am looking for a better solution. Is there something like 
a RemoveIdenticalStaves engraver where I could define which staves may 
be removed?

Regards
Helge