code: func (ch *Channel) NotifyReturn(c chan Return) chan Return {
  	ch.notifyM.Lock()
  	defer ch.notifyM.Unlock()
  
  	if ch.noNotify {
  		close(c)
  	} else {
  		ch.returns = append(ch.returns, c)
  	}
  
  	return c
  }
docs: /*
  NotifyReturn registers a listener for basic.return methods.  These can be sent
  from the server when a publish is undeliverable either from the mandatory or
  immediate flags.
  
  A return struct has a copy of the Publishing along with some error
  information about why the publishing failed.
  
  */