anchor.diff:          : _performInterpolation(node.value, trim: true);
   
       if (node.children == null) {
  -      _addChild(new CssAtRule(node.name, value: value, span: node.span));
  +      _parent.addChild(new CssAtRule(node.name, value: value, span: node.span));
       }
   
       _withParent(new CssAtRule(node.name, value: value, span: node.span), () {
  
positive.code:   CssValue<Value> _performExpression(Expression expression) =>
        new CssValue(expression.accept(_expressionVisitor));
  
    void _addChild(CssNode node) {
      _parent.addChild(node);
    }
  
    /*=T*/ _withParent/*<S extends CssParentNode, T>*/(
        /*=S*/ node, /*=T*/ callback(),
        {bool through(CssNode node), bool removeIfEmpty: false}) {
  
negative.code:       return;
      }
  
      _parent.addChild(new CssDeclaration(name, cssValue, span: node.span));
    }
  
    void visitAtRule(AtRule node) {
  