Printing part-combine texts whenever a new line starts

[Reinhold Kainhofer]

In all commercial full scores the part-combine texts (a2, I, II, etc.) are 
printed again in each new line (on the first real note of the new line). I'm 
currently trying to implement this behavior in LilyPond, but I'm running into 
some problems.

1) I tried to do it similarly to the Clef_engraver, which acknowledges 
bar_line grobs. However, as those bar_line grobs are in the Staff context, the 
Part_combine_engraver (in the Voice context!) does not hear them. Is there any 
way for an engraver in the Voice context to receive those bar_line events?

2) The alternative would be to use a PartCombineTextSpanner, which has other 
problems. In particular,
  a) the text should not always be printed immediately after a line break, 
     but potentially only when the first note in a new line occurs. How can 
     I tell a broken spanner to wait for a real note?
  b) I suspect that using part-combine text spanners will reserve vertical
     space, even if no line should ever be printed. I have real problems with 
     text crescendo spanners shifting up bar numbers, even if no spanner line
     is printed.

Any ideas how to fix any of these problems? Or do you have any idea how to 
tackle that problem from a different angle?

Thanks,
Reinhold
-- 
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Reinhold Kainhofer, address@hidden, http://reinhold.kainhofer.com/
 * Financial & Actuarial Math., Vienna Univ. of Technology, Austria
 * http://www.fam.tuwien.ac.at/, DVR: 0005886
 * LilyPond, Music typesetting, http://www.lilypond.org