A UGen which monitors another UGen to see when it is finished. Some UGens, such
as PlayBuf , RecordBuf , Line , XLine , EnvGen , Linen , BufRd ,
BufWr , DbufRd , and the Buffer delay UGens set a 'done' flag when they are
finished playing. This UGen echoes that flag as an explicit output signal when
it is set to track a particular UGen. When the tracked UGen changes to done, the
output signal changes from zero to one.
the UGen to track