code: public static void filterFields(File trainingCorpus, File newRawFile,
              int[] fieldsToKeep) throws IOException {
          FileTrainingCorpus ftc = new FileTrainingCorpus(trainingCorpus);
          Writer writer = new BufferedWriter(new FileWriter(newRawFile));
          Iterator<Document> iterator = ftc.iterator();
          while (iterator.hasNext()) {
              MultiFieldDocument doc = (MultiFieldDocument) iterator.next();
              String representation = doc.getStringSerialization(fieldsToKeep);
              writer.write(representation);
          }
          writer.close();
      }
docs: rewrite a raw file so that only a subset of the fields are kept