docs: def __set_due_date(self, value):
          '''
          Sets the due date of the invoice.
          @param value:date
          '''
          if type(value) != date:
              raise ValueError('Due date must be an instance of date.')
  
          if self.__date.date() and value < self.__date.date():
              raise ValueError("Due date cannot be anterior to the invoice " \
                               "date.")
  
          self.__due_date = value