code: func AddPreCheck(preCheck PreCheckFunc) ChallengeOption {
  	// Prevent race condition
  	check := preCheck
  	return func(chlg *Challenge) error {
  		chlg.preCheck.checkFunc = func(_, fqdn, value string, _ PreCheckFunc) (bool, error) {
  			if check == nil {
  				return false, errors.New("invalid preCheck: preCheck is nil")
  			}
  			return check(fqdn, value)
  		}
  		return nil
  	}
  }
docs: // AddPreCheck Allow to define checks before notifying ACME that the DNS challenge is ready.
  // Deprecated: use WrapPreCheck instead.