<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def modifications(self) -> List[Modification]: """ Return a list of modified files. :return: List[Modification] modifications """

        if self._modifications is None:
            self._modifications = self._get_modifications()

        return self._modifications