Weirdness in distribution of systems across pages

[Frank Steinmetzger]

Hello list,

I have written a document with a number of pieces within a single \book block. 
Each piece is a separate \score. I'm now working on an even density of the 
systems, so the document looks nice as a whole. I have one problem here:

For some pieces I have set a fixed system-count to override lilypond’s 
default, also I have a \pageBreak between all pieces except between 2 and 4. 
This yields the result on the left of the screenshot. It's almost perfect, 
except for piece 9, which is only three systems big with (relatively) much 
space in between. So I tried if it could be 4 systems long and it can.

However, for some reason, when I do so it messes up piece 2. Although it is 
two manual \pageBreaks apart, it is influenced by this one system-count and 
its systems are spread across one additional page (though its system count 
remains the same). You can see the result on the right column of the screenie.


Each piece is stored in a separate file. Those piece files contain the music 
in the form of

\partone=\new ChoirStaff
<<
                \staffone
                \lyricstostaffone

                \stafftwo
                \lyricstostafftwo

                etc...
>>

without any additional \layout information, only the occasional \break. The 
main file has the scores and directly \includes the pieces into them.

Do you have any idea why piece 2 is influenced by piece 9? And -- more 
importantly -- a way to solve this mystery?
-- 
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noel.ly
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screenie.png
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