Support to merge piece object with missing instruments?

[neldivad]

a = mp.chord('A4, E4, G4, B4') % (1/16, 1/16)
b = mp.chord('B4, E4, G4, B4') % (1/4, 1/4)
c = mp.chord('C4, E4, G4, B4') % (1/2, 1/2) 

t1 = mp.track(a*8, instrument=1)
t2 = mp.track(b*2, instrument=6)
t3 = mp.track(c, instrument=7)

part1 = mp.build(
    [t1, t2, t3], bpm=30 # 3 track piece
)

part2 = mp.build(
    [t1, t2], bpm=30, # 2 track piece
)

Suppose I add 2 tracks

(part1 + part2).play() # ok 
(part2 + part1).play() # not ok

The issue is that part2 starts with 2 instruments, so the beginning of the next piece would introduce a new instrument that is not initialized, thus failing the join operator.

The solution is to add a dummy track

a = mp.chord('A4, E4, G4, B4') % (1/16, 1/16)
b = mp.chord('B4, E4, G4, B4') % (1/4, 1/4)
c = mp.chord('C4, E4, G4, B4') % (1/2, 1/2) 

t1 = mp.track(a*8, instrument=1)
t2 = mp.track(b*2, instrument=6)
t3 = mp.track(c, instrument=7)

t4 = mp.track(mp.chord(''), instrument=7) # empty track

part1 = mp.build(
    [t1, t2, t3], bpm=30
)

part2 = mp.build(
    [t1, t2, t4], bpm=30, 
)

(part2 + part1).play() # now ok