def binfiles_set(self, isnap):
possible_files = set(self.filename(fstem, isnap, force_legacy=True)<EOL>for fstem in phyvars.FIELD_FILES)<EOL>return possible_files & self.files<EOL>
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.