code: func (m *MemoryBackend) Unsubscribe(client *Client, topics []string, ack Ack) error {
  	// delete subscriptions
  	for _, t := range topics {
  		client.Session().(*memorySession).subscriptions.Empty(t)
  	}
  
  	// call ack if provided
  	if ack != nil {
  		ack()
  	}
  
  	return nil
  }
docs: // Unsubscribe will delete the subscription.