docs: 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