Merge rests engraver

New engraver for combining rests of the same duration in different voices.
From the message in
http://www.mail-archive.com/lilypond-user%40gnu.org/msg69608.html

"It doesn't kill any grobs so

the chance of segfaults is lower. Do overlapping grobs have any other
unintended consequences?"


I don't know Scheme or its interface to LilyPond well enough to know from
the code.  In case it isn't clear, I'm not the author.  Jay Anderson is.

Knute Snortum
(via Gmail)


On Sun, Jun 22, 2014 at 2:37 PM, Janek Warchoł <notifications@github.com>
wrote:

> In editorial-tools/merge-rests-engraver/README.md:
>
> > @@ -0,0 +1,16 @@
> > +Merge Rests Engraver
> > +====================
> > +
> > +`merge-rests-engraver` and `merge-mmrests-engraver` are used to combine two rests of the same duration from two voices into one.  This eliminates the need to add `\once \omit` to the rest in one of the voices.  `merge-mmrests-engraver` combines whole measure and multi-measure rests and `merge-rests-engraver` combines all others.
>
> From the output of the example it seems that the resulting rests are
> indeed \oneVoice. However, a clarification in README would be welcome
> anyway.
>
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