forced bream overridden by acciatura?

[Helge Kruse]

I am typesetting a modern piece with a lot of meter changes. 
Additionally I want to keep the original layout while I am editing the 
score. Therefor I defined a timeline and a breaks block. I got a mystic 
warning that puzzled me:

warning: forced break was overridden by some other event, should you be 
using bar checks?

Bar checks did not gave any help. I experimented with block comments to 
identify the line(s) that caused the warning. Finally I found that this 
warning vanishes when I don't use the \acciatura after the \break.

- Can I keep my concept of manual breaks and avoid this warning? The 
acciatura is part of the music and should not be removed.
- Is there a way to isolate this without try and error?

Regards,
Helge


\version "2.15.38"


timeline = {
  %49
  \time 12/8    s8*12 | \bar "||"
  %50
  \time 9/8    s8*9 | s8*9 | s8*9 |
}

breaks = {
  %49
  s8*12 |
  s8*9 | \break
}

up = \relative c' {
  %49
  dis4. e fis g |
  %50 9/8
  a! g fis |
  \acciaccatura{b8}a4. g fis8 g a |
% without  \acciaccatura there is no warning:
%  a4. g fis8 g a |
}

\score {
  \new Staff=up { << \timeline \breaks \up >> }
}