As the part combiner shuffles musical content between Voice contexts, it can place notes and rests in a Voice with an ongoing multi-measure rest. The part combiner currently addresses this by killing multi-measure rests at certain times; a major shortcoming of which is that once a rest is killed, it can never reappear.
This enhancement to the Multi_measure_rest_engraver will allow future changes to the part combiner which kill multi-measure rests less often (hopefully never).
https://codereview.appspot.com/265160043/
Passes make, make check and a full make doc.
Patch on countdown for September 17th
I've postponed my planned changes that depended on this patch, so there is no reason to commit it. I do have some more ideas for improving multi-measure rests when voices share a staff, so maybe it will come back into play soon.