code: func (mem *memNamedMapping) Watch(subscriber string, callback func(idxmap.NamedMappingGenericEvent)) error {
  	mem.Debug("Watch ", subscriber)
  
  	_, found := mem.subscribers.LoadOrStore(subscriber, callback)
  	if found {
  		return fmt.Errorf("Already registered channel per subscriber ")
  	}
  	return nil
  }
docs: // Watch allows to subscribe for tracking changes in the mapping.
  // When an item is added or removed, the given <callback> is triggered.