anchor.diff:        this.result = result;
       } else {
         this.result = new Set();
  -      this.resutt.add(null);
  +      this.result.add(null);
       }
     }
   }
  
positive.code: 
    bool isNullable() => this.first.isNullable() && this.second.isNullable();
    bool isEmpty() => this.first.isNullable() || this.second.isNullable();
    Set parseNull(){
      Set result = new Set();
      for ( var l in this.first.parseNull()) {
        for (var r in this.second.parseNull())
  
negative.code:   
    bool isNullable() => true;
    bool isEmpty() => false;
    Set _parseNull() => this.result;
    Parser _derive(token) => new Empty();
  
    /** singleton factory */
    static Null _instance;
  