anchor.diff:    validL, validR: Boolean;
     refObj: TSliceGeometryICLRObject;
   begin
  +  widthL := Double.NaN;
  +  widthR := Double.NaN;
     Result := gtsFailed; // sentinel
     if Assigned(fPalette) then
     begin
  
positive.code: end;
  
  function TSliceDiffGeometryIC.GenerateTileCalc(const aExtent: TExtent; aBitmap: FMX.Graphics.TBitmap; aPixelWidth, aPixelHeight: Double): TGenerateTileStatus;
  begin
    // todo: implement
    Result := gtsFailed;
  end;
  
  { TSliceDiffGeometryICLR }
  
negative.code: begin
    // recalc extent
    fMaxExtent := fCurrentSlice.fMaxExtent.Intersection(fRefSlice.fMaxExtent);
    fDataVersion := fDataVersion+1; // trigger new set of tiles in cache
    fLayer.signalRefresh(timeStamp);
  end;
  
  