code: private String ubPrefixFeatureString(String feat)
    {
      if (feat.endsWith("|C")) {
        return "U-" + feat;
      } else if (feat.endsWith("|CpC")) {
        return "B-" + feat;
      } else {
        return feat;
      }
    }
docs: Prefix features with U- (for unigram) features
  or B- (for bigram) features
  @param feat String representing the feature
  @return new prefixed feature string