code: func (runner *runner) AddEntry(entry string, set *IPSet, ignoreExistErr bool) error {
  	args := []string{"add", set.Name, entry}
  	if ignoreExistErr {
  		args = append(args, "-exist")
  	}
  	if _, err := runner.exec.Command(IPSetCmd, args...).CombinedOutput(); err != nil {
  		return fmt.Errorf("error adding entry %s, error: %v", entry, err)
  	}
  	return nil
  }
docs: // AddEntry adds a new entry to the named set.
  // If the -exist option is specified, ipset ignores the error otherwise raised when
  // the same set (setname and create parameters are identical) already exists.