docs: public void printFeatures(String exportFile, Collection<List<IN>> documents) {
      try {
        PrintWriter pw = IOUtils.getPrintWriter(exportFile);
        for (List<IN> doc:documents) {
          String str = getFeatureString(doc);
          pw.println(str);
        }
        pw.close();
      } catch (IOException ex) {
        throw new RuntimeException(ex);
      }
    }