docs: def emit(self, data_frame):
          """Use this function in emit data into the store.
  
          :param data_frame: DataFrame to be recorded.
          """
          if self.result is not None:
              raise MultipleEmitsError()
          data_frame.columns = [self.prefix + '__' + c
                                for c in data_frame.columns]
          self.result = data_frame