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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 files(self):
 """Set of found binary files output by StagYY."""

        if self._rundir['ls'] is UNDETERMINED:
            out_stem = pathlib.Path(self.par['ioin']['output_file_stem'] + '_')
            out_dir = self.path / out_stem.parent
            if out_dir.is_dir():
                self._rundir['ls'] = set(out_dir.iterdir())
            else:
                self._rundir['ls'] = set()
        return self._rundir['ls']