code: func Flag(fs *flag.FlagSet) {
  	for _, id := range All {
  		desc := id.Flag()
  		key := fmt.Sprintf("X%s", id.Flag())
  		fs.Var(&idValue{X: id}, key, desc)
  	}
  }
docs: // Flag configures the given FlagSet with the flags to configure
  // all active experiments.