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),

			return fmt.Errorf("reverse select %v wanted %v. got %v.", key, want, got)
		}
		return nil
	}
}

func emptyMap(m *BiMultimap) error {
	if len(m.labeledObjects) != 0 {
		return fmt.Errorf("Found %v labeledObjects. Wanted none.", len(m.labeledObjects))
	}
	if len(m.selectingObjects) != 0 {
		return fmt.Errorf("Found %v selectingObjects. Wanted none.", len(m.selectingObjects))
	}
	if len(m.labeledBySelecting) != 0 {