private static void FormulaChangedCallback(DependencyObject d, 
            DependencyPropertyChangedEventArgs args)
        {
            var formulaStringConverter = new FormulaStringConverterBootstrap();

            ((FormulaButton) d).TextBlockFormula.Text = (string)
                formulaStringConverter.Convert(args.NewValue, typeof(FormulaButton), null, "");
            //throw new NotImplementedException();
        }
        private static void FormulaChangedCallback(DependencyObject d,
                                                   DependencyPropertyChangedEventArgs args)
        {
            var formulaStringConverter = new FormulaStringConverterBootstrap();

            ((FormulaButton)d).TextBlockFormula.Text = (string)
                                                       formulaStringConverter.Convert(args.NewValue, typeof(FormulaButton), null, "");
            //throw new NotImplementedException();
        }