anchor.diff:    }
   
     bool attemptWord(String word) {
  +    if (words.contains(word)) {
  +      return false;
  +    }
  +    
       if (_wordIsValid(word)) {
         score += scoreForWord(word);
         words.add(word);
  
positive.code:   List selectedPositions = [];
    int score = 0;
    final Dictionary dictionary;
    List<String> words = <String>[];
  
    final CanvasElement canvas;
    final ImageAtlas letterAtlas;
  
negative.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,
  