code: func (n *NotifyGroup) Notify() {
  	n.l.Lock()
  	defer n.l.Unlock()
  	for ch := range n.notify {
  		select {
  		case ch <- struct{}{}:
  		default:
  		}
  	}
  	n.notify = nil
  }
docs: // Notify will do a non-blocking send to all waiting channels, and
  // clear the notify list