public Lexicon(List<Tree<String>> trainTrees) {
      for (Tree<String> trainTree : trainTrees) {
        List<String> words = trainTree.getYield();
        List<String> tags = trainTree.getPreTerminalYield();
        for (int position = 0; position < words.size(); position++) {
          String word = words.get(position);
          String tag = tags.get(position);
          tallyTagging(word, tag);
        }
      }
    }