anchor.diff:  part of xml;
   
   /**
  + * Mixin to serialize XML to a [StringBuffer].
  + */
  +abstract class _XmlWritable {
  +
  +  /**
  +   * 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);
  +
  +}
  +
  +/**
    * Abstract XML node.
    */
  -abstract class XmlNode extends Iterable<XmlNode> {
  +abstract class XmlNode extends Iterable<XmlNode> with _XmlWritable {
   
     XmlNode _parent;
   
  
positive.code: /**
   * XML entity name.
   */
  class XmlName {
  
    final String _prefix;
    final String _local;
  
negative.code:   String get prefix => _prefix;
    String get qualified => toString();
  
    void writeTo(StringBuffer buffer) {
      if (prefix != null) {
        buffer.write(prefix);
  