anchor.diff:      }
       return element;
     }
  -
  -  void _addElementAttributes(Element element) {
  -
  -  }
   }
   
   /// Parses horizontal rules like `---`, `_ _ _`, `*  *  *`, etc.
   class HorizontalRuleSyntax extends BlockSyntax {
     RegExp get pattern => _RE_HR;
  +
  +  const HorizontalRuleSyntax();
   
     Node parse(BlockParser parser) {
       final match = pattern.firstMatch(parser.current);
  
positive.code: /// Parses atx-style headers: `## Header ##`.
  class HeaderSyntax extends BlockSyntax {
    RegExp get pattern => _RE_HEADER;
  
    Node parse(BlockParser parser) {
      final match = pattern.firstMatch(parser.current);
  
negative.code:   bool get canEndBlock => false;
  
    String get listTag;
  
    const ListSyntax();
  
    Node parse(BlockParser parser) {
      final items = <ListItem>[];
  