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