code: def id2rel(self, xs):
          
          if isinstance(xs, list):
              return [self._id2rel[x] for x in xs]
          return self._id2rel[xs]
docs: Map id(s) to relation(s)
  
          Parameters
          ----------
          xs : int
              id or a list of ids
  
          Returns
          -------
          str or list
              relation or a list of relations