the "perfect" break-engraver (was Re: [Feature Request] \compressFullBarRests improvement(s))

[Kieren MacMillan]

Further to this thread…

> I just think it — or something like it — should be vetted and [the 
> improved/approved version] included with the regular distro.

I think the “perfect” break-engraver would admit coding like the following 
(pseudocode; all numbers in measures):

    \lineBreakAt 27
    \lineBreaksAt (12 28 34 48)
    \noLineBreaks (27-30)
    \lineBreaksForbidAt 28
    \lineBreaksAllowAt 34
etc., and identical for \pageBreaks-

These breaking-codes would be put in the \layout block of a score — cf. the way 
David N’s engraver currently works — so that there is no need to mix this 
presentation-layer information into the content layer.

The only additional [optional] argument I can think of is a Staff (or Voice?) 
"context target”, e.g.,

    \lineBreakAt 27 #”soprano"

which would be necessary for polymetric music (where the measures might not all 
end simultaneously).

Thoughts?

Cheers,
Kieren.