anchor.diff:    List selectedPositions = [];
     int score = 0;
     final Dictionary dictionary;
  -  List<String> words = <String>[];
  +  Set<String> words = new Set<String>();
   
     final CanvasElement canvas;
     final ImageAtlas letterAtlas;
  
positive.code: @observable
  class Game {
  
    static const DIMENSIONS = 4;
    static const Map<String, num> LETTERS =  const {
      'A': 1, 'B': 3, 'C': 3, 'D': 2, 'E': 1,
  
negative.code:   }
  
    bool attemptWord(String word) {
      if (words.contains(word)) {
        return false;
      }
      
      if (_wordIsValid(word)) {
        score += scoreForWord(word);
        words.add(word);
  