code: func NewExpirationStore(keyFunc KeyFunc, expirationPolicy ExpirationPolicy) Store {
  	return &ExpirationCache{
  		cacheStorage:     NewThreadSafeStore(Indexers{}, Indices{}),
  		keyFunc:          keyFunc,
  		clock:            clock.RealClock{},
  		expirationPolicy: expirationPolicy,
  	}
  }
docs: // NewExpirationStore creates and returns a ExpirationCache for a given policy