code: func Tee(s1, s2 Sinker) Sinker {
  	fn := func() chan<- Report {
  		d1 := s1.Sink()
  		d2 := s2.Sink()
  		u := make(chan Report)
  		go tee(u, d1, d2)
  		return u
  	}
  
  	return SinkFunc(fn)
  }
docs: // Tee works like Unix tee; it forwards all progress reports it receives to the
  // specified sinks