docs: @Override
      public List<Tree> next() {
          List<Tree> ret = new ArrayList<>();
          try {
              for (int i = 0; i < batchSize; i++)
                  if (hasNext())
                      ret.addAll(treeVectorizer.getTreesWithLabels(sentenceIterator.nextSentence(),
                                      sentenceIterator.currentLabel(), labels));
          } catch (Exception e) {
              throw new RuntimeException(e);
          }
  
          return ret;
      }