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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 binfiles_set(self, isnap):
 """Set of existing binary files at a given snap. Args: isnap (int):
 snapshot index. Returns: set of pathlib.Path: the set of output files available for this snapshot number. """

        possible_files = set(self.filename(fstem, isnap, force_legacy=True)
                             for fstem in phyvars.FIELD_FILES)
        return possible_files & self.files