added fr.is_good() to check existence of alleged files

        return f"{len(self)} alleged, " \

               f"{len(self) - sum(self.to_table()['existence'])} missing, " \

               f"{sum(self.to_table()['existence'])} to db"

    def is_good(self):

        """ Check if all alleged files exist. """

        return all(self.to_table()['existence'])