type watchTracker struct {
	// indexes represents a set of registered indexes.
	// It can't change after creation.
	indexes builtinIndexes

	lock       sync.Mutex
	watchCount map[watchIdentifier]int
}

func NewWatchTracker() WatchTracker {
	return &watchTracker{
		indexes:    getBuiltinIndexes(),
		watchCount: make(map[watchIdentifier]int),
	}
}

const (
	unsetValue = "<unset>"
)