<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def delete_flag(self):
 """ Sets this message as un flagged """

        self.__status = Flag.NotFlagged
        self.__start = None
        self.__due_date = None
        self.__completed = None
        self._track_changes()