docs: public void printFeatures(String exportFile, int[][][][] docsData, int[][] labels)  {
      try {
        PrintWriter pw = IOUtils.getPrintWriter(exportFile);
        for (int i = 0; i < docsData.length; i++) {
          for (int j = 0; j < docsData[i].length; j++) {
            StringBuilder sb = new StringBuilder();
            int label = labels[i][j];
            sb.append(classifier.classIndex.get(label));
            for (int k = 0; k < docsData[i][j].length; k++) {
              for (int m = 0; m < docsData[i][j][k].length; m++) {
                String feat = classifier.featureIndex.get(docsData[i][j][k][m]);
                feat = ubPrefixFeatureString(feat);
                sb.append(delimiter);
                sb.append(feat);
              }
            }
            pw.println(sb.toString());
          }
          pw.println();
        }
        pw.close();
      } catch (IOException ex) {
        throw new RuntimeException(ex);
      }
    }