public MathNode AddMathNode(Type type)
        {
            MathNode node = mathExp.AddMathNode(type);

            mathExp.AddVariableMapItem(node);
            if (_editor != null)
            {
                _editor.ShowVariableMap(mathExp.VariableMap);
            }
            this.Refresh();
            _changed = true;
            return(node);
        }