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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def to_maildir(self, flags: Iterable[Union[bytes, Flag]]) -> str: """Return the string of letter codes that are used to map to defined IMAP flags and keywords. Args: flags: The flags and keywords to map. """

        codes = []
        for flag in flags:
            if isinstance(flag, bytes):
                flag = Flag(flag)
            from_sys = self._from_sys.get(flag)
            if from_sys is not None:
                codes.append(from_sys)
            else:
                from_kwd = self._from_kwd.get(flag)
                if from_kwd is not None:
                    codes.append(from_kwd)
        return ''.join(codes)