anchor.diff:      return null;
     }
   
  -  /**
  -   * Answer a print string of the receiver.
  -   */
  -  String toString() {
  -    var buffer = new StringBuffer();
  -    writeTo(buffer);
  -    return buffer.toString();
  -  }
  -
  -  /**
  -   * Writes the XML string of the receiver to a {@code buffer}.
  -   */
  -  void writeTo(StringBuffer buffer);
  -
   }
   
   /**
  
positive.code: part of xml;
  
  /**
   * Abstract XML node.
   */
  abstract class XmlNode extends Iterable<XmlNode> {
  
    XmlNode _parent;
  
  
negative.code:   String get prefix => _prefix;
    String get qualified => toString();
  
    void writeTo(StringBuffer buffer) {
      if (prefix != null) {
        buffer.write(prefix);
  