anchor.diff:      Result:= StrToFloat(aValue);
   end;
   
  -class function TNullableDouble.VariantToString(const aValue: Variant; const aDigits : byte): String;
  +class function TNullableDouble.VariantToString(const aValue: Variant; const aDisplayFormat: string): String;
   var
     i : integer;
     str : String;
  
positive.code: constructor TNullableDouble.Create();
  begin
    inherited Create;
    FDigits := 0;
  end;
  
  constructor TNullableDouble.Create(aValue: Double);
  begin
    inherited Create;
    Value := aValue;
  end;
  
  
  procedure TNullableDouble.Assign(const aSource: TNullableDouble);
  begin
    Self.FDigits:= aSource.Digits;
    if not aSource.IsNull then
      Self.Value := aSource.Value
    else
      Self.IsNull := true;
    FTagChanged:= false;
  
negative.code:     Result := Self.Value;
  end;
  
  function TNullableDouble.AsFormattedString(const aFormat: String): String;
  begin
    if Self.IsNull then
      Result := ''
  