docs: public int generateAt(Instance instance, IFeatureAlphabet features, int pos, int... numLabels) throws Exception {
          String feaOri = getFeaString(instance, pos, numLabels);
          if (feaOri == null)
              return -1;
          int index = -1;
          if (keymap.containsKey(feaOri))
              index = features.lookupIndex(keymap.get(feaOri), (int) Math.pow(numLabels[0], order + 1));
          return index;
      }