box.go
func (b *RefCountBox) Free() {
	b.Lock()
	defer b.Unlock()

	b.count--
	if b.count == 0 {
		b.db.Close()
		b.db = DB{}
	}
}
func (b *RefCountBox) Free() {
	b.Lock()
	defer b.Unlock()

	b.count--
	if b.count == 0 {
		b.db.Close()
		b.db = DB{}
	}
}
// Free decreases the reference count. It close the DB if the count reaches
// zero.