code: func (pc PAConfig) CheckChallenges() error {
  	if len(pc.Challenges) == 0 {
  		return errors.New("empty challenges map in the Policy Authority config is not allowed")
  	}
  	for name := range pc.Challenges {
  		if !core.ValidChallenge(name) {
  			return fmt.Errorf("Invalid challenge in PA config: %s", name)
  		}
  	}
  	return nil
  }
docs: // CheckChallenges checks whether the list of challenges in the PA config
  // actually contains valid challenge names