mux sync.RWMutex

	// Objects.
	labeledObjects   map[Key]*labeledObject
	selectingObjects map[Key]*selectingObject

	// Associations.
	labeledBySelecting map[selectorKey]*labeledObjects
	selectingByLabeled map[labelsKey]*selectingObjects
}

// NewBiMultimap creates a map.
func NewBiMultimap() *BiMultimap {
	return &BiMultimap{
		labeledObjects:     make(map[Key]*labeledObject),