code: def append_id(expr, id_col='append_id'):
      
      if hasattr(expr, '_xflow_append_id'):
          return expr._xflow_append_id(id_col)
      else:
          return _append_id(expr, id_col)
docs: Append an ID column to current column to form a new DataFrame.
  
      :param str id_col: name of appended ID field.
  
      :return: DataFrame with ID field
      :rtype: DataFrame