@Override
 public void onFetchResult(final Result<FetchedItem<OriginalInput>, OriginalInput> result) {
   fetch.add(result);
 }
 @Override
 public void onProcessingResult(final Result<FetchedItem<OriginalInput>, Output> result) {
   processing.add(result);
 }
 public ResultStatistics build() {
   return new ResultStatistics(fetch.build(), processing.build(), hasListenerDelegationFailures);
 }