anchor.diff:    assertAttributeInvariant(xml);
     assertTextInvariant(xml);
     assertIteratorInvariants(xml);
  +  assertPrettyPrinting(xml);
   }
   
   void assertDocumentInvariant(XmlNode xml) {
  
positive.code:     ancestors.removeLast();
    }
    check(xml);
  }
  
  void main() {
  
negative.code: void validate(String input) {
    var tree = parse(input);
    assertTreeInvariants(tree);
    var copy = parse(tree.toXmlString());
    expect(tree.toXmlString(), copy.toXmlString());
  }
  
  void assertTreeInvariants(XmlNode xml) {
  