Set a calculated value for this Expression.
        Used when writing formulas using XlsxWriter to give cells
        an initial value when the sheet is loaded without being calculated.

def value(self):
        """Set a calculated value for this Expression.
        Used when writing formulas using XlsxWriter to give cells
        an initial value when the sheet is loaded without being calculated.
        """
        try:
            if isinstance(self.__value, Expression):
                return self.__value.value
            return self.__value
        except AttributeError:
            return 0