performer=klass()
function performer:init(track,channel,instrument,time,roots)
 assert_range(#roots,1,33,"#roots")

 self.track=track
 self.channel=channel
 self.instrument=instrument
 self.time=time
 self.roots=roots
 self.selected_root=1
 self.offset=0
end

function performer:root(root)
 assert_range(root,1,#self.roots,"root")
 self.selected_root=root
end
function performer:play(duration,note,verbs)
 local base=self.roots[self.selected_root]
 local note=base+note+self.offset
 local t0=self.time
 local t1=t0+duration
 self.time+=duration
 self.instrument:spackle(self.track,self.channel,t0,t1,note,verbs)
end
function performer:rest(duration)
 self.time+=duration
end