<part-combiner> collects in one place the options for routing parts to Voice contexts and generating marks.</part-combiner>
\partcombine, \partcombineUp, and \partcombineDown accept an optional <part-combiner> in place of an optional chord range, though this is not yet documented.</part-combiner>
This makes customization significantly easier than before.
Patch updated to avoid changing the interface of \partcombine etc.
https://codereview.appspot.com/265260043/
Passes make, make check and a full make doc.
Patch on countdown for October 2nd.
There seems to be some discussion here, I'll set this back to review for now.
I need time to digest the review comments.