code: public NGram createNGram(List words) {
      int[] nums = new int[words.size()]; //ngram needs it own copy of this array so keep making a unique one for each instance.
      for (int wi=0;wi<nums.length;wi++) {
        nums[wi] = wordMap.getIndex(words.get(wi));
        if (nums[wi] == -1) {
          return null;
        }
      }
      return new NGram(nums);
    }
docs: Creates a new n-gram for the specified list of words.
  @param words The words which are part of this n-gram.
  @return the newly created object or null if the n-gram contains an unknown word.