code: func (bus *EventBus) Unsubscribe(topic string, handler interface{}) error {
  	bus.lock.Lock()
  	defer bus.lock.Unlock()
  	if _, ok := bus.handlers[topic]; ok && len(bus.handlers[topic]) > 0 {
  		bus.removeHandler(topic, bus.findHandlerIdx(topic, reflect.ValueOf(handler)))
  		return nil
  	}
  	return fmt.Errorf("topic %s doesn't exist", topic)
  }
docs: // Unsubscribe removes callback defined for a topic.
  // Returns error if there are no callbacks subscribed to the topic.