bus index to write to. For an audio-rate UGen, this is an audio-bus, for a control-rate UGen, this is a control-bus.
signal to write to the bus. If the UGen is audio-rate, the input must also be audio-rate.
cross-fade value. The new bus contents will be
old_bus_content * (1 - xfade) + in * xfade
bus index to write to.
signal to write to the bus.
Abstract method which must be implemented by creating the actual