Re: Merge_rests_engraver: fix vertical rest positions (issue 334740043 by address@hidden)

[lilypond]

Thanks for the suggestions, I’ll apply these fixes, test them, and add a
regression test.

There’s another issue (already present in the “old”
Merge_rests_engraver): Single-bar or non-compressed MMRs in 4/2 time are
positioned incorrectly because the engraver assumes they are semibrevis
rests. I will fix that too.

https://codereview.appspot.com/334740043/