<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def accounts(self) -> AccountsAggregate: """ Returns the Accounts aggregate """

        if not self.__accounts_aggregate:
            self.__accounts_aggregate = AccountsAggregate(self.book)
        return self.__accounts_aggregate