internal override XElement CreateXml()
        {
            var xRoot = new XElement(XmlConstants.Brick);

            xRoot.SetAttributeValue(XmlConstants.Type, XmlConstants.XmlChangeVariableBrickType);

            var xElement = VariableFormula.CreateXml();

            xElement.SetAttributeValue(XmlConstants.Category, XmlConstants.VariableChange);

            var xFormulalist = new XElement(XmlConstants.FormulaList);

            xFormulalist.Add(xElement);

            xRoot.Add(xFormulalist);

            xRoot.Add(UserVariable.CreateXml());

            return(xRoot);
        }