code: func (b *RefCountBox) Free() {
  	b.Lock()
  	defer b.Unlock()
  
  	b.count--
  	if b.count == 0 {
  		b.db.Close()
  		b.db = DB{}
  	}
  }
docs: // Free decreases the reference count. It close the DB if the count reaches
  // zero.