anchor.diff:  }
   
   class _UndefinedExpression extends ExpressionBox {
  -  Element element;
  +  final Element element;
   
     _UndefinedExpression._internal(this.element);
   
  
positive.code: }
  
  class _ConditionExpression extends ExpressionBox {
    Expression? expression;
    bool value;
  
    _ConditionExpression(this.expression, {this.value = true});
  
  
negative.code:       environment.whereType<_UndefinedExpression>();
  
    void _recursiveGetExpressions(
        List<Expression> expressions, Iterable<Element?> elements, bool? value) {
      for (var element in environment.reversed) {
        if (element.haveToStop(elements)) {
          return;
        }
        if (element is _ConditionExpression && element.value == value) {
          var expression = element.expression;
          if (expression != null) {
            expressions.add(expression);
          }
        }
      }
      outer?._recursiveGetExpressions(expressions, elements, value);
  