part combiner question

[Russ Ross]

I've played around with the \partcombine function but I haven't found
a way to achieve the result I'm looking for.  I'd like to combine two
parts with one as the "upper" voice and the other as the "lower" such
that the distinction is clear in the final result.  This seems to be a
common style for vocal music (I see it a lot for hymns) and also for
settings with Violin I & II parts on the same line, etc.  In
particular, here's the result I'm looking for:

* When the "lower" part is below the "upper" part, the stems are
joined in the same direction (unless the interval between them is
excessively large) and beams are shared when possible.
* When the "lower" part is in unison with or above the "upper" part,
the "lower" part is printed with a stem going down and the "upper"
part with a stem going up.  As a special case of this rule, unison
notes print a single notehead with two stems, one up and one down.

The first part seems to be what \partcombine does, but I haven't found
a convenient way to do the second part.  Is there a way to do this
that I've overlooked?  My results so far have only achieved one or the
other of the two rules listed above, but not both.

Thanks,

Russ