docs: public void printFullFeatureMatrix(PrintWriter pw) {
      String sep = "\t";
      for (int i = 0; i < featureIndex.size(); i++) {
        pw.print(sep + featureIndex.get(i));
      }
      pw.println();
      for (int i = 0; i < labels.length; i++) {
        pw.print(labelIndex.get(i));
        Set<Integer> feats = new HashSet<Integer>();
        for (int j = 0; j < data[i].length; j++) {
          int feature = data[i][j];
          feats.add(Integer.valueOf(feature));
        }
        for (int j = 0; j < featureIndex.size(); j++) {
          if (feats.contains(Integer.valueOf(j))) {
            pw.print(sep + '1');
          } else {
            pw.print(sep + '0');
          }
        }
      }
    }