code: func (s *Subscription) Delivered() (int64, error) {
  	if s == nil {
  		return -1, ErrBadSubscription
  	}
  	s.mu.Lock()
  	defer s.mu.Unlock()
  	if s.conn == nil {
  		return -1, ErrBadSubscription
  	}
  	return int64(s.delivered), nil
  }
docs: // Delivered returns the number of delivered messages for this subscription.