anchor.diff:  
     static XmlNode _renderRSS2(Feed feed) => '';
   
  +  Person author;
     String id;
     String title;
     String description;
     Uri link;
     Uri image;
     String copyright;
  -  String feed;
  -  String hub;
  +  Uri feed;
  +  Uri hub;
     DateTime updated;
  -  String author;
     List<Item> items = [];
     List<String> catagories = [];
  -  List<String> contributors = [];
  +  List<Person> contributors = [];
   
     String toXmlString({
         renderer: FeedRenderer.atom,
  
positive.code: import 'package:xml/xml.dart';
  
  class Item {
    String title;
  
negative.code:           ..element('title', nest: feed.title);
  
          // Build recommended elements.
          if (feed.author != null) {
            builder.element('author', nest: () {
              builder.build('name', nest: feed.author.name);
  
              if (feed.author.email != null)
                builder.build('email', nest: feed.author.email);
  
              if (feed.author.link != null)
                builder.build('uri', nest: feed.author.link.toString());
            });
          }
  
          if (feed.link != null) {
            builder.element('link', attributes: {
              'rel': 'alternate',
              'href': feed.link.toString()
            });
          }
  
          if (feed.feed != null) {
            builder.element('link', attributes: {
              'rel': 'self',
              'href': feed.feed.toString()
            });
          }
  
          if (feed.hub != null) {
            builder.element('link', attributes: {
              'rel': 'hub',
              'href': feed.hub.toString()
            });
          }
  
          // Build optional elements.
          if (feed.description != null)
            builder.element('subtitle': nest: feed.description);
  
          if (feed.image != null)
            builder.element('logo', nest: feed.image.toString());
  
          if (feed.copyright != null)
            builder.element('rights': feed.copyright);
  
          for (String catagory in feed.catagories)
            builder.element('catagory', attributes: {'term': catagory});
  
          for (Person contributor in feed.contributors) {
            builder.element('contributor', nest: () {
              builder.build('name', nest: contributor.name);
  
              if (contributor.email != null)
                builder.build('email', nest: contributor.email);
  
              if (contributor.link != null)
                builder.build('uri', nest: contributor.link.toString());
            });
          }
  
        });
  