<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add_flag(self, flag):
 """ Add flag to the flags and memorize this attribute has changed so we can regenerate it when outputting text. """

        super(Entry, self).add_flag(flag)
        self._changed_attrs.add('flags')