code: def to_maildir(self, flags: Iterable[Union[bytes, Flag]]) -> str:
          
          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)
docs: 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.