Multiple staves
Each **kern spine in the data will produce a staff in the graphical
notation. The lowest staff is the left-most spine in the data, and
the highest staff in the notation is the right-most spine.
Here is a more complicated example where the number of voices changes on each staff:
Note that when the subspines for adjacent staves merge at the same
time (at the end of measure 2), then the merges must be staggered
for grouping clarity. Also when a spine needs to expand from one
to three voices, there must be two *^ expansions (the first to
go from one to two subspines, and the second to go from two to three
spines). Also note the use of 2ryy, which is used to delay merging
of the spines until the end of the measure (otherwise the two right
subspines could merge together before merging with the first subspine
for the staff at the end of the measure). Also note that subspines
should not be ended before the full duration of the last note in
the subspine (so the first two subspines in the top staff in measure
2 can not merge half-way through the measure since the note in
the first subspine has not yet finished sounding).