make score be line break aware

[Francisco Vila]

Hello.
Is this possible? I want to typeset something (like a spanner or a
different height of the lyrics baseline) that I want to last only
until it reaches the next line break.

If only I could explain it well, it would be a sort of intelligent
tweak that 'knows' when a line break has happened and it then kills
himself. There would be as liquid layout as possible meaning that
layout does not interfere with content, i.e. making an explicit \break
would not be an optimal solution.

BTW this is about my project with alternate lyrics. The original score
I have does this because the mid-hight lyrics baseline (after the two
alternatives) is not needed anymore after the line break. I could
attach a scan if you want.

Thank you!
-- 
Francisco Vila. Badajoz (Spain)
http://www.paconet.org