anchor.diff:  
     MySet parseNull() {
       var r = new MySet();
  -    print('$this: $r');
       return r;
     }
     Parser derive(token) {
       var r = new Empty();
       return r;
  -    print('($token) => $this: $r');
     }
   
     /// singleton factory
  
positive.code:     parseNull_called = false;
      parseNull_value = new MySet();
      cache = new Map<dynamic, Parser>();
      print('done');
    }
  
    get nullable {
  
negative.code: 
    MySet parse(List tokens){
      var parser = this;
      var pass = 0;
      for( var token in tokens){
        pass += 1;
        print('PASS: $pass');
        parser = parser.derive(token);
      }
      print('Done parsing');
      return parser.parseNull();
    }
  }
  