anchor.diff:  
   void assertCopyInvariants(XmlNode xml) {
     final copy = xml.copy();
  -  assertParentInvariants(xml);
     assertParentInvariants(copy);
  -  assertNameInvariants(xml);
  +  assertNameInvariants(copy);
  +  assertCompareInvariants(xml, copy);
  +}
  +
  +void assertTransformerInvariants(XmlNode xml) {
  +  final copy = XmlTransformer().visit(xml);
  +  assertParentInvariants(copy);
     assertNameInvariants(copy);
     assertCompareInvariants(xml, copy);
   }
  
positive.code:   assertTextInvariants(xml);
    assertIteratorInvariants(xml);
    assertCopyInvariants(xml);
    assertPrintingInvariants(xml);
  }
  
  
negative.code:   assertTextInvariants(xml);
    assertIteratorInvariants(xml);
    assertCopyInvariants(xml);
    assertTransformerInvariants(xml);
  }
  
  void assertDocumentInvariants(XmlNode xml) {
  