code: def check_id_idx_exclusivity(id, idx):
      
      if (id is not None and idx is not None):
          msg = ("'id' and 'idx' fields can't both not be None," +
                 " please specify subset in only one of these fields")
          logger.error(msg)
          raise Exception("parse_gctx.check_id_idx_exclusivity: " + msg)
      elif id is not None:
          return ("id", id)
      elif idx is not None:
          return ("idx", idx)
      else:
          return (None, [])
docs: Makes sure user didn't provide both ids and idx values to subset by.
  
      Input:
          - id (list or None): if not None, a list of string id names
          - idx (list or None): if not None, a list of integer id indexes
  
      Output:
          - a tuple: first element is subset type, second is subset content