code: func MustRegisterCmd(method string, cmd interface{}, flags UsageFlag) {
  	if err := RegisterCmd(method, cmd, flags); err != nil {
  		panic(fmt.Sprintf("failed to register type %q: %v\n", method,
  			err))
  	}
  }
docs: // MustRegisterCmd performs the same function as RegisterCmd except it panics
  // if there is an error.  This should only be called from package init
  // functions.