code: def subset(self, used_indices, params=None):
          
          if params is None:
              params = self.params
          ret = Dataset(None, reference=self, feature_name=self.feature_name,
                        categorical_feature=self.categorical_feature, params=params,
                        free_raw_data=self.free_raw_data)
          ret._predictor = self._predictor
          ret.pandas_categorical = self.pandas_categorical
          ret.used_indices = used_indices
          return ret
docs: Get subset of current Dataset.
  
          Parameters
          ----------
          used_indices : list of int
              Indices used to create the subset.
          params : dict or None, optional (default=None)
              These parameters will be passed to Dataset constructor.
  
          Returns
          -------
          subset : Dataset
              Subset of the current Dataset.