anchor.diff:    assertIteratorInvariants(xml);
     assertCopyInvariants(xml);
     assertTransformerInvariants(xml);
  +  assertVisitorInvariants(xml);
   }
   
   void assertDocumentInvariants(XmlNode xml) {
  
positive.code:   assertCompareInvariants(xml, copy);
  }
  
  void assertCompareInvariants(XmlNode original, XmlNode copy) {
    expect(original, isNot(same(copy)),
        reason: 'The copied node should not be identical.');
  
negative.code:   assertIteratorInvariants(xml);
    assertCopyInvariants(xml);
    assertTransformerInvariants(xml);
    assertVisitorInvariants(xml);
    assertPrintingInvariants(xml);
  }
  
  