anchor.diff:    // toString() => 'Reduce($parser)';
   
     Parser derive(token) {
  -    print('_ ($token) => $this');
       var r = new Reduce(parser.derive(token), this.reduction);
  -    print('_ ($token) => $this: $r');
       return r;
     }
   
     MySet parseNull(){
  -    print('_ $this');
       MySet result = new MySet();
       MySet parseNull = parser.parseNull();
   
       for( final r in parseNull){
         result.add(reduction(r));
       }
  -    print('_ $this: $result');
       return result;
     }
   }
  
positive.code: 
    MySet parseNull() {
      var r = new MySet();
      print('$this: $r');
      return r;
    }
    Parser derive(token){
  
negative.code:     if (this.token == token){
        r = new Null(new MySet.from([token]));
      }
      return r;
    }
  }
  