anchor.diff:    aRegExp,
     aMIMEMessage,
     aSelfDescribeCBOR,
  -  anArrayElement,
     aMapValue,
     aMapKey,
     nothing
  
positive.code:   /// Used to indicate what the
    /// next decoded item should be.
    whatsNext _next = whatsNext.nothing;
  
    void onInteger(int value) {
      // Do not add nulls
  
negative.code:     }
    }
  
    void onMap(int size) {
      final DartItem item = new DartItem();
      item.type = dartTypes.dtMap;
      item.data = new List<dynamic>();
      item.targetSize = size;
      _append(item);
      _next = whatsNext.aMapKey;
    }
  
    void onTag(int tag) {
      // Switch on the tag type
  