@Override
  public Optional<T> oneOrNone() {
    List<T> list = Publishers.blockingListGet(fetcher.oneOrNone());
    Preconditions.checkState(list.size() <= 1, "Invalid list size: %s", list.size());
    return list.isEmpty() ? Optional.empty() : Optional.of(list.get(0));
  }
Implementation of {@link SyncFetcher}. Also has some package-private methods
to allow delegation.
criteria/common/src/org/immutables/criteria/repository/sync/SyncFetcherDelegate.java