Setter for **self.__checked** attribute.

        :param value: Attribute value.
        :type value: bool

def checked(self, value):
        """
        Setter for **self.__checked** attribute.

        :param value: Attribute value.
        :type value: bool
        """

        if value is not None:
            assert type(value) is bool, "'{0}' attribute: '{1}' type is not 'bool'!".format("checked", value)
        self.set_checked(value)