My original intent was that you'd use a "tied-to" staff for multiple voices on one (visible) staff -- duke.cmn for example. But, that's not very convenient when these voices come and go. The next fallback would be to use the explicit slur list makers (slur-, -slur-, and -slur), but I just noticed there's a bug in them that screws up in the current example -- I'll add it to my list...