docs: public List<Pair<int[][][], int[]>> documentsToDataAndLabelsList(Collection<List<IN>> documents) {
      int numDatums = 0;
  
      List<Pair<int[][][], int[]>> docList = new ArrayList<Pair<int[][][], int[]>>();
      for (List<IN> doc : documents) {
        Pair<int[][][], int[]> docPair = documentToDataAndLabels(doc);
        docList.add(docPair);
        numDatums += doc.size();
      }
  
      System.err.println("numClasses: " + classIndex.size() + ' ' + classIndex);
      System.err.println("numDocuments: " + docList.size());
      System.err.println("numDatums: " + numDatums);
      System.err.println("numFeatures: " + featureIndex.size());
      return docList;
    }