有人可以幫助我理解這些代碼嗎？在client-go項目中，有一些代碼我無法理解。代碼路徑為\tols\cache\store.go    Add(obj interface{}) error    Update(obj interface{}) error    Delete(obj interface{}) error    List() []interface{}    ListKeys() []string    Get(obj interface{}) (item interface{}, exists bool, err error)    GetByKey(key string) (item interface{}, exists bool, err error)    // Replace will delete the contents of the store, using instead the    // given list. Store takes ownership of the list, you should not reference    // it after calling this function.    Replace([]interface{}, string) error    Resync() error}type cache struct {    // cacheStorage bears the burden of thread safety for the cache    cacheStorage ThreadSafeStore    // keyFunc is used to make the key for objects stored in and retrieved from items, and    // should be deterministic.    keyFunc KeyFunc}var _ Store = &cache{}最后一行“var _ Store = &cache{}”，這是什么意思，有官方文檔支持嗎？