Alternative partcombine strategy possible?

[Abel Cheung]

Does anybody have experience about overriding existing partcombine
methods? I have a score that behave like the one below:

- When both voices have identical rests, use partcombineUnisonoOnce
- Use partcombineApart for everything else

In the snippet below, the second line represents what I intended to
do, but manually overriding each rest for several hundred bars isn't
quite practical. Is there any possibility of using custom value for
PartCombineForceEvent forced-type property? Or there's any other
elegant method that helps saving note input time?

==========================
upper = \relative c' { c4 r8 d r4 e8 d c2 r2 }
lower = \relative c' { c4 r8 b c d e b c2 r2 }

upperClumsy = \relative c' {
\partcombineApart c4
\partcombineUnisonoOnce r8
d r4 e8 d c2
\partcombineUnisonoOnce r2
}

\partcombine { \partcombineApart \upper } \lower
\partcombine \upperClumsy \lower % <--- desired
==========================

-- 
Abel Cheung
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