code: func handleDebugLevel(s *rpcServer, cmd interface{}, closeChan <-chan struct{}) (interface{}, error) {
  	c := cmd.(*btcjson.DebugLevelCmd)
  
  	// Special show command to list supported subsystems.
  	if c.LevelSpec == "show" {
  		return fmt.Sprintf("Supported subsystems %v",
  			supportedSubsystems()), nil
  	}
  
  	err := parseAndSetDebugLevels(c.LevelSpec)
  	if err != nil {
  		return nil, &btcjson.RPCError{
  			Code:    btcjson.ErrRPCInvalidParams.Code,
  			Message: err.Error(),
  		}
  	}
  
  	return "Done.", nil
  }
docs: // handleDebugLevel handles debuglevel commands.