docs: private void validateAndTriggerCallback(
        final List<KeyValue> kvs, 
        final Map<byte[], List<Annotation>> annotations,
        final List<SimpleEntry<byte[], List<HistogramDataPoint>>> histograms) {
  
      countdown.countDown();
      final long count = countdown.getCount();
      if (kvs.size() > 0) {
        kv_map.put((int) count, kvs);
      }
      
      for (final byte[] key : annotations.keySet()) {
        final List<Annotation> notes = annotations.get(key);
        if (notes.size() > 0) {
          // Optimistic write, expecting unique row keys
          annotation_map.put(key, notes);
        }
      }
      
      if (histograms.size() > 0) {
        histMap.put((int) count, histograms);
      }
      
      if (countdown.getCount() <= 0) {
        try {
          mergeAndReturnResults();
        } catch (final Exception ex) {
          results.callback(ex);
        }
      }
    }