code: func NewRefCache(refresh time.Duration) *lazycache.Cache {
  	initializer := func(key lazycache.Key) (interface{}, error) {
  		ck, ok := key.(CacheKey)
  		if !ok {
  			return nil, errors.New("unexpected cache key")
  		}
  		return NewRef(refresh, ck.Context(), ck.ChConfigRef()), nil
  	}
  
  	return lazycache.New("Membership_Cache", initializer)
  }
docs: // NewRefCache a cache of membership references that refreshed with the
  // given interval